双介孔硅基材料负载Pd催化剂上甲苯氧化性能的研究

催化氧化技术是挥发性有机物(VOCs)减排与控制的主流技术之一,其关键之处在于高效催化材料的研究与开发,负载型贵金属催化材料由于其低温下优越的VOCs催化氧化性能,受到国内外研究者的广泛关注.对于负载型催化剂而言,载体的性质直接影响活性相的分散,反应物和生成物的扩散与吸脱附,是影响负载型催化剂性能的主要因素.近年来,多级孔结构硅基材料由于具有多级的孔道结构、高比表面积和大的孔体积,逐渐成为VOCs催化氧化材料的研究热点.本文采用溶胶凝胶法和浸渍法制备了系列双介孔结构硅基材料负载Pd催化剂(Pd/BMS-x),通过控制合成过程中氨水的用量以调节催化剂的介孔结构分布. X射线衍射(XRD)结果表明,所合成的Pd/BMS-x催化剂在~2.0°的衍射峰,类似于MCM-41的(100)晶面衍射峰,表明所有的样品均具有有序的介孔结构. N2吸脱附实验表明所有样品的比表面积均高于1000m2/g,孔径分布表明Pd/BMS-30样品为单一介孔结构,而Pd/BMS-5~Pd/BMS-20样品具有2.64 nm以及18–45 nm范围内的双介孔结构,且Pd/BMS-15样品介孔分布较为集中. Pd/BMS-x催化剂上甲苯催化氧化性能测试表明,双介孔结构的Pd/BMS-5~Pd/BMS-20催化剂上甲苯催化氧化活性远高于单一介孔结构的Pd/BMS-30催化剂,表明载体结构对催化剂性能有重要影响.其中, Pd/BMS-15催化剂性能最佳(T90为228°C)且具有较强的稳定性,250°C条件下,反应持续60h催化剂未见明显失活. SEM和TEM结果表明, Pd/BMS-15催化剂中Pd高度分散于载体上,平均粒径在~3 nm左右.而Pd/BMS-30催化剂中Pd颗粒间有明显的团聚,平均粒径在8~17 nm之间.分散度测试表明,单一介孔结构的Pd/BMS-30催化剂, Pd分散度仅为27%,而双介孔结构Pd/BMS-5–Pd/BMS-20催化剂介于39%到69%,其中Pd/BMS-15催化剂中Pd分散度高达69%.与常规单一介孔MCM-41和MCM-48负载Pd催化剂相比,在低空速(42000 h–1)条件下, Pd/BMS-15催化剂上甲苯催化氧化性能与Pd/MCM-41和Pd/MCM-48催化剂相当.高空速(70000h–1)条件下, Pd/BMS-15催化剂的活性远高于单一介孔的Pd/MCM-41和Pd/MCM-48催化剂. Pd/BMS-15催化剂独特的双介孔结构,有利于活性相Pd的分散、反应物的扩散和传输,特别是在高空速条件下,有利于反应物与活性相的接触,提高了材料的氧化反应性能.进一步考察了材料的水热稳定性,将11 vol%的水蒸气引入到反应体系中,测试结果表明水蒸气的加入导致Pd/MCM-41和Pd/MCM-48催化剂的甲苯催化氧化性能显著下降,反应500 min后甲苯转化率分别从100%下降到76%和81%,而对于Pd/BMS-15催化剂,水蒸气的引入并未导致其活性明显下降,从而表明Pd/BMS-15催化材料具有较高的水热稳定性.     

用烟雾箱研究甲苯与OH自由基光化学反应

文中用自制的烟雾腔系统研究了甲苯与OH自由基的光化学反应,得到的结论是其在大气中的主要产物是苯醛、甲基苯酚和苯硝酸甲脂.甲苯在烟雾箱内的寿命约为60分钟.     

THE ADSORPTION BEHAVIORS OF TOLUENE ON HYPERCROSSLINKED RESINS

Investigation of the adsorption characteristics of toluene on two hypercrosslinked resins. The static and the kinetic adsorption were studied. The equilibrium adsorption date was fitted to freundlich adsorption isotherm models to evaluate the model parameters. The enthalpy, free energy, entropy are indicative of an exothermic, a spontaneous and disorter decreasing process. Experimental results show that the two adsorbents are better than the Amberlite XAD-4 for removingthe toluene in aqueous solutions.     

近红外光谱法测定有机混合物中的正己烷、环己烷、甲苯的含量

用近红外光谱法对具有不同含量水平的环己烷、正己烷、甲苯、苯的四氯化碳混合溶液中的环己烷、正己烷、甲苯进行了定量分析。对于体积百分含量在1.4％～20％之间的环己烷,校正集真值与预测值相关系数r=0.9969,RSD=0.34％;对于体积百分含量在0.04％～0.96％的正己烷,校正集真值与预测值相关系数r=0.9999,RSD=0.83％;对于含量在26.0～259.8mg·L~(-1)的甲苯,校正集真值与预测值相关系数r=0.9921,RSD=4.63％。近红外光谱可以检测到26.0mg·L~(-1)的含量水平,其预测值为25.9mg·L~(-1),相对误差为0.38％。结果表明,用近红外光谱快速、准确、同时测定不同含量水平的组份可以获得较理想的结果。     

HZSM-5催化甲苯和甲醇烷基化反应机理的密度泛函理论研究

对二甲苯(PX)是重要的有机化工原料,主要用于生产对苯二甲酸(PTA)和对苯二甲酸二甲酯(DMT), PTA和 DMT可经缩聚生产化纤、合成树脂和塑料等聚酯产品. PX主要通过甲苯歧化、二甲苯异构化或甲苯与 C9芳烃烷基转移等方式生产.由于三种二甲苯和乙苯的沸点接近,需要经过吸附分离或深冷分离才能得到高纯度的 PX,传统工艺物料循环量大,设备庞大,操作费用高.而通过甲苯和甲醇烷基化反应直接高选择性生成 PX,可大大降低成本,具有非常高的经济效益和研究价值.自1970年代以来,国内外众多科研院所对甲苯和甲醇烷基化催化剂进行了广泛研究,但催化剂选择性和稳定性仍需进一步提高.为了加深对甲苯和甲醇烷基化反应的认识,指导催化剂开发,有必要对甲苯和甲醇烷基化生成二甲苯的反应机理进行深入研究.当前甲苯和甲醇烷基化机理研究主要存在以下问题：(1)计算得到的能量多为电子能,而非自由能;(2)所采用的模型多为团簇模型,使用 ONIOM方法,对长程作用力描述不充分;(3)认为甲苯只有一种吸附状态;(4)没有考虑偕烷基化反应.本文采用周期性模型,通过密度泛函理论研究了 HZSM-5分子筛上甲苯和甲醇烷基化反应机理,通过计算熵得到了反应自由能,并考虑了偕烷基化反应.由于甲基的存在,在甲苯的吸附态中,甲基会伸向孔道的不同方向,因此我们认为甲苯有多种吸附态,而不同的吸附态会生成不同的二甲苯.结果表明,甲苯可以在对位、间位、邻位和偕位上通过协同机理或分步机理发生烷基化反应.在协同机理中,甲苯在对位、间位、邻位和偕位发生烷基化反应的自由能垒分别为167,138,139和183 kJ/mol.在分步机理中,甲醇脱水生成甲氧基的自由能垒为145 kJ/mol,是决速步骤;而甲苯和甲氧基对位、间位、邻位和偕位烷基化的自由能垒分别为127,105,106和114 kJ/mol.两种机理中 PX的生成能垒均比 MX和 OX高,与文献报道的结果不同.文献均认为, PX的生成能垒最低.一方面这可能是由于所采用模型的不同,本文采用周期性模型,能更充分考虑长程作用力的影响;另一方面可能是由于对甲苯吸附态的不同处理,我们认为甲苯有多种吸附态,不同的吸附态会生成不同的二甲苯,而文献均只考虑了一种甲苯吸附态.但是,在实验中, PX选择性最高.这可能是由于：(1) PX在 HZSM-5孔道的扩散速率比 MX和 OX高2–3个数量级;(2)甲苯和甲醇烷基化生成的 MX和OX迅速发生异构化反应生成 PX,异构化反应速率高于甲苯烷基化速率.两种机理中, C8H11+都是重要的中间物种,它可以反馈一个质子给分子筛骨架,生成二甲苯;也可以脱烷基生成甲烷和乙烯等气相产物.研究发现,甲烷的生成是由于 C8H11+物种中的一个 H质子从苯环上的碳原子转移到甲基上的碳原子造成的,计算得到的对位、间位和邻位 C8H11+生成甲烷的能垒分别为136,132和134 kJ/mol.由于十元环孔道的限制, HZSM-5孔道中很难通过甲苯歧化反应生成苯;偕烷基化生成的碳正离子有可能脱烷基生成乙烯和乙烷等产物,进而生成苯.碳正离子脱烷基反应生成了大量气相产物,造成反应液收降低.碳正离子脱烷基反应与甲醇制烯烃过程的烃池机理相一致,因此甲苯和甲醇烷基化反应也遵循烃池机理.     

Catalytic characterization of pillared clays through toluene methylation reaction

Pillared clays (PILCs) with Al and Zr oxide pillars were studied in terms of their structure and texture. The catalytic properties of the samples were evaluated through toluene methylation reaction. As comparison material, a commercial zeolite HZSM-5 was used. The toluene conversion at short time-on-stream over PILCs, although smaller than the value obtained with HZSM-5, attains values of 37 (molar %). In all cases the reaction products are a mixture of xylene, ethyltoluene and trimethylbenzene isomers. All samples present higher selectivity to xylene isomers, which are the primary products. For Zr pillared clay, the percentage of these isomers is the highest.     

PERVAPORATION SEPARATION FOR TOLUENE/n-HEPTANE MIXTURE BY POLYIMIDE MEMBRANES CONTAINING FLUORINE

Five kinds of polyimides were synthesized using five dianhydrides (including 2,2-bis[4-(3,4-dicarboxyphenoxy)- phenyl] propane dianhydride (BPADA),3,3',4,4'-diphenylsulfone-tetracarboxylic dianhydride (DSDA),4,4'- (hexafluoroisopropylidene)-diphthalic anhydride (6FDA),1,4-bis(3,4-dicarboxyphenoxy) benzene dianhydride (HQDPA), and 4,4'-oxydiphthlic dianhydride (ODPA)) and 2,2-bis[4-(4-aminophenoxy)phenyl] hexafluoropropane (BDAF) via the two- step method that included polyaddition to form the polyamic aci...     

Liquid–liquid equilibria for pseudo-ternary systems: (Sulfolane + 2-ethoxyethanol) + octane + toluene at 293.15 K

Liquid–liquid equilibrium data, both binodal and tie lines are presented for the pseudo-ternary systems: {(sulfolane + 2-ethoxyethano) (1) + octane (2) + toluene (3)} at 293.15 K. The experimental liquid–liquid equilibrium data have been correlated using NRTL and UNIQUAC models, and the binary interaction parameters of these components have been presented. The correlated tie lines have been compared with the experimental data. The comparisons indicate that both NRTL and UNIQUAC models satisfactorily correlated the equilibrium compositions. The tie-line data of the studied systems also were correlated using the Hand method.     

Measurements of the isochoric heat capacity,the critical point (TC, ρC) and vapor–liquid coexistence curve (TS, ρS) of high-purity toluene near the critical point

Isochoric heat capacities (C_V, V, T), phase boundary properties (T_S, ρ_S) and the critical (T_C, ρ_C) parameters for high-purity (0.9999+ mole fraction) toluene have been measured with a high temperature, high pressure, nearly constant volume adiabatic calorimeter and quasi-static thermogram technique. Measurements were made at three selected liquid and vapor isochores 777.8, 555.25, and 214.64 kg m⁻³ in the temperature range from 379 to 591 K. For five near-critical isochores 268.68, 281.68, 296.62, 301.52, and 318.28 kg m⁻³, the measurements were made in the immediate vicinity of the coexistence curve in order to accurately determine the phase transition temperatures (T_S, ρ_S) (shape of the coexistence curve near the critical point) and the critical parameters (T_C, ρ_C). The total combined uncertainty of heat capacity, density, and temperature measurements were estimated to be less than 2%, 0.06%, and 15 mK, respectively. The uncertainties reported in this paper are expanded uncertainties at the 95% confidence level with a coverage factor of k = 2. The uncertainty of the phase transition and the critical temperature value was 0.02 K. The Krichevskii parameter for some toluene-containing binary mixtures was calculated. The derived values of the Krichevskii parameter were used to estimate the effect of dilute impurities on the critical parameters of toluene. The measured values of saturated density near the critical point were interpreted in terms of the “complete scaling” theory in order to study singularity behavior of the coexistence curve diameter. The measured isochoric heat capacities and saturated densities were compared with the data reported by other authors and values calculated from an equation of state and other correlations.