1 Introduction
Formaldehyde is an important chemical intermediate and product1,2, and the production of formaldehyde by non-oxidative direct dehydrogenation of methanol has attracted widespread attention by academia and industry3,4. No greenhouse gas CO2 is produced during the process of dehydrogenation of methanol with anhydrous formaldehyde obtained. In general, the fixed bed reactors with catalyst particles stacked randomly are used for dehydrogenation of methanol5. In this case, internal and external diffusion effects cannot be avoided at the scale of one catalyst particle owing to the limitation of mass transfer and heat transfer6, which can weaken the catalytic performance. Besides, the fluid channel between the catalyst particles is uneven due to a large number of particles stacked randomly, leading to the uneven flowing of the reactant across the catalyst particles. The uneven flowing in the catalytic bed will cause non-uniform distribution of residence time, temperature field and concentration field, which can further weaken the catalytic performance7,8. Especially, the local porosity of the catalytic bed is higher near the wall of the reactor and a channeling phenomenon is prone to existed9, which could make the non-uniform field distribution more serious. The non-uniform distribution of the field and the residence time would cause the formaldehyde converting into by-products such as CO, HCOOH, and HCOOCH34,10 .
Membranes with micro or nano scale pores are good carriers for catalyst nanoparticles immobilization11. The catalytic membrane nano reactor (CMNR) with deep-permeation nanocomposite structure (DPNS) would be obtained if the nanoparticles are in situ immobilized in membrane pores along the thickness direction of the membrane. Multi-scale synergistic enhancement for catalysis can be achieved in CMNR during the subsequent dehydrogenation of methanol. (1) At the nanoparticles scale, the catalysts are immobilized in the membrane pores without being shaped. In this case, the reactant fluid can contact with all nanoparticles via flowing instead of diffusion12, which could increase the effective contact area between catalysts and reactant fluid, leading to the internal diffusion effect in conventional fixed bed reactors being eliminated. (2) At the scale of membrane pores, the catalytic reaction is confined within the membrane pores, which can reduce the thickness of the mass transfer boundary layer13, improve the mass transfer efficiency, and eliminate the external diffusion effect in the conventional fixed bed reactor14. (3) At the scale of the catalytic bed layer, a sheet of membrane is designed as a catalytic bed layer, and good flowing uniformity can be achieved owing to the uniform disperse of the membrane pores. Therefore, the uniform distribution of residence time, concentration field and temperature field will be achieved15. Moreover, the multi catalytic bed layers can be set up in series with each layer controlled independently. Therefore, the precise control of temperature within the whole reactor can be achieved during the catalysis process.
In our previous study, it has been found that the catalytic nanoparticles can be in situ immobilized in membrane pores by flowing synthesis16-20. During this process, the precursor fluid for nanoparticles formation can permeate through the membrane pores under the external driven force. Several advantages can be realized by flowing synthesis (1) The catalytic nanoparticles can be immobilized in each membrane pore and distributed evenly along the thickness direction of the membrane. Under the external driven forces, the precursor fluid can overcome the surface tension of the liquid and flow in each membrane pores, which ensures the catalytic nanoparticles immobilized in each membrane pore; (2) Rapid mixing of the precursor and enhanced mass transfer and heat transfer can be achieved in the confined space of the membrane pores with micro or nano scale, which could promote the nanoparticles formation and structure regulation21; (3) According to the principle of deep filtration in porous media, the catalytic nanoparticles can be immobilized stably in tortuous membrane pores during flowing synthesis process under the effect of inertial force, Brownian motion and other factors.
In the field of methanol dehydrogenation, Cu and ZnO nanoparticles were performed high activity and remarkable selectivity to formaldehyde4,22-24. It has been reported that the yield of formaldehyde could be achieved at 60% under 650 ℃ with the catalysts containing Cu and Zn. In this work, porous Titanium membrane (Ti membrane) with good thermal conductivity, thermal stability and mechanical properties was used as the substrate. In order to increase the loading amount of Cu/ZnO nanoparticles immobilized in membrane pores, the Ti membrane substrate was firstly treated by heat alkali and silanization modified with amino groups grafted in membrane pores for metal ion chelation before Cu/ZnO nanoparticles immobilized. After that, the zeolitic imidazolate framework-8 (ZIF-8) was in situimmobilized in the treated Ti membrane substrate by flowing synthesis, then the Cu2+ was introduced by ion-exchange between Cu2+ and Zn2+ in ZIF-8. Finally, Cu/ZnO nanoparticles were immobilized in the membrane pores by calcining and reducing to fabricate the Cu/ZnO/Ti CMNR. During the experiments of methanol dehydrogenation reaction, the effect of reaction temperature, reactant gas flux and the number of membranes setting up in series has been determined in CMNR.