优质期刊论文发表网提供专业的论文发表、论文写作以及期刊推广服务!QQ:820771224

电话:18796337551
当前位置:优质期刊论文发表网材料论文 → 文章正文

Mg-Zn复合氧化物催化剂制备、表征及应用

作者:优质期刊论文发表网  来源:www.yzqkw.com  发布时间:2019/10/24 9:58:16  

摘要:本文采用共沉淀法制备Mg-Zn复合氧化物催化剂,将其用于大豆油甘油解合成单甘酯(MAG)。着重考察焙烧温度对Mg-Zn复合氧化物催化剂结构及催化单甘酯合成反应性能的影响。对不同焙烧温度下的Mg-Zn复合氧化物催化剂进行XRD、BET、SEM、HRTEM等表征。使用MDIJade软件对XRD数据进行计算,得到Mg-Zn复合氧化物催化剂的结构参数、催化剂中MgO、ZnO晶胞中晶胞参数及键长键角值等随焙烧温度变化的规律,并与SEM和HRTEM的数据进行对比,结果较为吻合。采用Hammett指示剂法测定了催化剂碱强度及碱量。结果表明:Mg-Zn复合氧化物催化剂适宜的焙烧温度为800 ℃,大豆油转化率达到96.5%,MAG收率为53.4 %。由XRD数据,不同焙烧温度下,MgO、ZnO晶胞发生晶格畸变,晶胞参数以及键长发生变化,形成Mg-Zn-O和Zn-Mg-O晶格,引起催化剂碱性发生变化。由BET得到700 ℃、800 ℃和900 ℃时,催化剂比表面积减小,孔径增大,有利于反应物分子进入孔道,充分反应。900℃介孔结构坍塌,催化剂活性降低。由HRTEM得到,800 ℃时Mg-Zn复合氧化物催化剂催化剂出现六边形结构,晶格间距发生变化,可能是由于Zn-Mg-O晶格的存在。

然后,对Mg/Zn摩尔比、焙烧温度、焙烧时间(三因素三水平)进行正交试验,优化Mg-Zn复合氧化物催化剂制备条件。结果表明,根据正交试验得到的最优催化剂制备条件为A2B3C1(即A2=Mg/Zn摩尔比为10,B3=焙烧温度为820 ℃,C1=焙烧时间为330min),MAG收率达到51.8 %。

另外,对Mg-Zn复合氧化物催化剂用于大豆油甘油解反应工艺条件进行了优化,通过考虑大豆油转化率、MAG收率、MAG选择性,结合产物与催化剂分离情况以及产物色泽,氧化性,气味等多方面问题,得到Mg-Zn复合氧化物催化剂催化大豆合成MAG的最优工艺条件。结果为:反应时间为2 h,反应温度为230 ℃,催化剂用量为0.45%(占大豆油质量),醇油摩尔比为2.5。此时,大豆油转化率为98.8%,MAG收率为58.7%,MAG选择性为59.4%。

对甘油三酯(TAG)和甘油二酯(DAG)建立了一种新的定量分析方法,该方法采用Platisil NH2正相柱,以正己烷:异丙醇(V:V)=10:1为流动相和溶剂。TAG在0.5~30mg/mL范围内线性良好,DAG在0.5~20 mg/mL范围内线性良好。同时采用高碘酸氧化-化学滴定法对MAG和甘油(Gly)进行了分析。然后,对Mg-Zn复合氧化物催化剂用于大豆油甘油解进行了部分动力学分析。首先,分析了系统的传质阻力。通过控制搅拌速率和不同颗粒尺寸排除内外扩散阻力的影响。采用1stOpt软件对甘油解可逆反应的动力学模型与实验动力学数据进行模拟,得到甘油解反应的动力学参数。结果表明,搅拌速率为600 rpm,催化剂颗粒无需筛分,可有效忽略内外扩散传质阻力。反应接近4 h以后,反应基本完全,MAG收率达到64%左右。各基元反应速率常数为k1=0.034,k2=0.0023,k3=0.7533,k4=0.0738,k5=0.0105。即反应控速步骤为DAG和甘油反应转化为MAG。忽略k6值,且k2值相比其他数值也可近似忽略,即TAG几乎不会通过逆反应而得到。

In this paper, a Mg-Zn mixed oxide catalystwas prepared by co-precipitation method, which was used to catalyze thesynthesize of monoglyceride (MAG) with soybean oil and glycerol. The effect ofcalcination temperature on the structure of Mg-Zn mixed oxide catalyst and thecatalytic performance of MAG synthesis were investigated and verified by XRD,BET, SEM and HRTEM. By using MDI Jade software to process XRD data, thestructural parameters of the Mg-Zn mixed oxide catalyst, the variation of thecell parameters as well as bond length angles of MgO and ZnO in the catalystwith different calcination temperature were calculated. These findings were inline with what we observed from SEM and HRTEM. The basic strength and basicityamount of the catalyst were determined by the Hammett indicator method. In theend, the comprehensive results showed that the suitable calcination temperatureof Mg-Zn mixed oxide catalyst was 800 ℃,soybean oil conversion rate 96.5%, MAGyield 53.4%. From the XRD data, we could observe the lattice distortion of theMgO and ZnO unit cells with the change of calcination temperatures. The unitcell parameters and the bond length were also changed and as a result, formingnew Mg-Zn-O and Zn-Mg-O lattices. According to the BET results, from 700 ℃ to900 ℃, the specific surface area of the catalyst decreased while the porediameter increased. This was favorable for the reactant molecules to enter thepores and fully react. The mesoporous structure collapsed at 900 ℃ and thecatalyst activity decreased. It was observed by HRTEM image that a hexagonalstructure of the Mg-Zn mixed oxide catalyst was formed at 800 ℃ which couldlead to the change of lattice spacing, and therefore prove the existence ofZn-Mg-O lattice.

Then, an orthogonal test was performed onthe Mg/Zn molar ratio, the calcination temperature, and the calcination time(three factors and three levels) to optimize the preparation conditions of theMg-Zn mixed oxide catalyst. The results showed that, according to theorthogonal test, the optimum catalyst preparation conditions were A2B3C1 (i.e.,A2=Mg/Zn molar ratio was 10, B3=calcination temperature was 820 ℃,C1=calcination time was 330 min), and the MAG yield reached 51.8%.

In addition, the process conditions of theMg-Zn mixed oxide catalyst for soybean oil glycerol hydrolysis reaction wereoptimized by considering soybean oil conversion rate, MAG yield, MAGselectivity, separation of product and catalyst, product color and oxidation,odor and other problems, the optimal process conditions for the synthesis ofMAG by soybeans catalyzed by Mg-Zn mixed oxide catalyst. The results were asfollows: reaction time was 2 h, reaction temperature was 230℃, catalyst amountwas 0.45% wt . oil, and molar ratio of alcohol to oil was 2.5. At this time,the soybean oil conversion rate was 98.8%, the MAG yield was 58.7%, and the MAGselectivity was 59.4%.

A new analytical method was established forquantification of triglyceride (TAG) and diglyceride (DAG) by using a PlatisilNH2 normal phase column with n-hexane:isopropanol (V:V)=10:1 as flow phase andsolvent. TAG has good linearity in the range of 0.5~30mg/mL, and DAG has goodlinearity in the range of 0.5~20mg/mL. At the same time, chemical titrationwere used for analysis of MAG and glycerol (Gly). Then, a partial kineticanalysis of the Mg-Zn mixed oxide catalyst for soybean oil glycerolysis wascarried out. First, the mass transfer resistance of the system was analyzed.The effect of internal and external diffusion resistance was ruled out bycontrolling the agitation rate and different particle sizes. The kinetic modelof the reversible reaction of glycerolysis was fitted to the experimentalkinetic data using 1stOpt software to calculate the kinetic parameters of theglycerolysis system. The results showed that when the stirring rate was 600rpm, the catalyst particles do not need to be sieved.

关键词:Mg-Zn复合氧化物;焙烧温度;单甘酯;动力学分析

Mg-Zn mixed oxide catalyst;calcinationtemperture; monoglycerides; knitic analysi

联系方式

客服QQ 820771224
客服热线18796337551
网站地址 www.yzqkw.com
郑重承诺 高效,快速,包发表!
优质期刊论文发表网真诚欢迎新老客户的光临与惠顾!