Author: Rafael Alfredo Díaz RealPublisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
The kinetics of the vapor phase air oxidation of methanol to formaldehyde over molybdenum oxide catalysts, antimony oxide catalyst, and their mixtures (both supported and unsupported), at atmospheric pressure and different operating conditions, have been studied in a fixed-bed integral reactor heated by a fluidized sand bath. The effect of various process variables, namely the process temperature (T), the ratio of catalysts to feed flow rate or space time (W/F), and the ratio of methanol fed to air (R), on conversion and yield have been determined. A screening study at varying operating conditions was performed to determine the optimum composition of a Sb$\sb2\rm O\sb4$-MoO$\sb3$ mixture. On the basis of this study a catalyst containing 67% $\rm Sb\sb2O\sb4$-33% MoO$\sb3$ was selected for the detailed kinetic study of oxidation of methanol to formaldehyde. The operating conditions studied were as follows: temperature in the range 623 to 698 K, space times from 5 to 50 $\rm g\sb{cat}/mol\sb{CH\sb3OH}h\sp{-1},$ and methanol to air ratios in the range 0.04 to 0.10 mol$\rm\sb{CH\sb3OH}h\sp{-1}/mol\sb{air}h\sp{-1}.$ This catalyst proved to be highly active and selective to formaldehyde formation. Yields up to $\sim$100% were obtained. Best operating conditions found were obtained at a space time of 27.5 for a methanol/air ratio of 0.06 and a temperature of 698 K. The rate equation for the oxidation of methanol to formaldehyde was derived on the basis of a two-stage redox mechanism$$\eqalign{\rm CH\sb3OH\sb{(g)} + S\sb{ox}\ {\buildrel{k\sb1}\over{\to}}\ &\rm HCHO\sb{(g)} + H\sb2O\sb{(g)} + S\sb{red}\cr\rm O\sb{2\sb{(g)}} + &\rm S\sb{red}\ {\buildrel{k\sb2}\over{\to}}\ S\sb{ox}\cr}$$where S$\rm\sb{ox}$ represents an active site of lattice oxygen and S$\rm\sb{red}$ represents a reduced site of lattice oxygen. The rate equation for the temperature of 648 to 698 K which correlated the data was$$\rm r = {k\sb1P\sb{M}\over 1+{k\sb1P\sb{M}\over 2k\sb2P\sb{O\sb2}}}$$where k$\sb1$ and k$\sb2$ are the temperature dependent rate constants of steps one and two. The equations relating k$\sb1$ and k$\sb2$ with temperature were$$\eqalign{&\rm ln\ k\sb1 = -6.4039-{6.9153\times10\sp3\over T}\cr&\rm ln\ k\sb2 = -3.0154 + {1.8809\times10\sp3\over T}\cr}$$ Several spectroscopic and analytical techniques, viz, electron spin resonance (ESR), x-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), and adsorption studies were used to characterize the catalysts. The surface are of the catalyst used in the kinetic study was 6.1 m$\sp2$/g as determined by the BET method. A preliminary study of the Sb-Mo oxide mixture (load of $\sim$5 wt%) supported on Y zeolite was also carried out. Maximum yield obtained was comparable to that obtained with pure MoO$\sb3.$ A new catalyst has been developed that gave nearly 100% conversion and 100% yield. The industrial potential of this catalyst is very promising.