2.4 Analysis methods
The microstructure and morphological characteristics of the membranes were observed by field emission scanning electron microscope (FESEM; JEOL, JSM700F) with ETD detector at 15 KV. The component and distribution of elements in the membranes were observed by the energy spectrum analysis system (EDS; Oxford instrument, INCA X-Max50). The X-ray diffraction (XRD) patterns were recorded by an EMPYREAN diffractometer with Cu ray at 40 kv, 40 mA to analyze the structure of crystal. The valence states of elements were detected on an X-ray photoelectron spectrometer (XPS; ESCALab220i-XL) with Al target ray, and the energy step was 1 eV. The thermogravimetric analysis was performed on a thermal analysis system (TGA; TGA/DSC2) in Ar, and the temperature was increased from 30 ℃ to 650 ℃ with the heating rate of 10 ℃·min-1. The field emission transmission electron microscope and high-resolution transmission electron microscope (TEM and HRTEM; FEI, Tecnai G22 F20 S-TWIN) were used to observe the morphology of nanoparticles that immobilized in the membrane pores. The TEM and HRTEM samples were prepared by cleaning the CMNRs in ultrasound for 24 h, and the suspension was centrifuged at 8000 r·min-1for 15 min. Finally, the powder was dispersed in methanol after being washed with methanol for 3 times. The loading amount of Cu and Zn in the CMNR were analyzed based on a plasma spectrometer (ICP; Agilent 725ES).
The liquid and gas products at the outlet of the CMNR were analyzed by gas chromatography workstation (GC-9790 II, Zhejiang Fuli). The liquid products were detected by flame ionization detector (FID) using an LZP-930 column (30 m × 0.32 μm × 1.0 μm), and the temperature of the injection was set at 180 ℃, the temperature of column oven was set at 50 ℃, the temperature of the detector was set at 180 ℃. The carrier gas was nitrogen and its flowrate was 40 mL·min-1. The gas products were detected by thermal conductivity detector (TCD) using a TDX-01 column (2 m × 3 mm), and the temperature of the injection was set at 100 ℃, the temperature of column oven was set at 90 ℃, the temperature of the detector was set at 100 ℃. The carrier gas was nitrogen and its flowrate was 24 mL·min-1.
During the process of methanol dehydrogenation to formaldehyde in CMNR, the reaction equations can be described as follows according to the analyzed results of products (Figure S4):
Combing Eq. (1) and Eq. (2), the productivity of formaldehyde (PFA , mmol·h-1·g-1) could be described as follows:
The productivity of hydrogen (PH2 , mmol·h-1·g-1) was calculated as:
where CH2 was the volume fraction of hydrogen of the outlet, %; Vgas was the volume of gas that collected at the downward of the CMNR, mL; was the reaction time, h;gcat was the loading amount of Cu/ZnO nanoparticles immobilized in the CMNR, g.
The productivity of methyl formate (PMF ,mmol·h-1·g-1) was calculated as:
where CMF was the mass concentration of MF in the condensate, mg·g-1; m was the mass of the condensate, g; MMF was the molecular weight of MF, which was 60.05 g·mol-1.
The conversion efficiency of methanol (XMeOH , %) was calculated as:
where FMeOH was the flowrate of methanol vapor that permeated into the CMNR, mmol·h-1.
The selectivity of the formaldehyde (SFA , %) was calculated as:
The selectivity of the methyl formate (SMF , %) was calculated as:
The catalyst turnover rate (TOF , mmol·h-1·g-1) was calculated as:
The formaldehyde production capacity was defined as the production of per membrane thickness of CMNR (Y FA, mmol·h-1·mm-1) and it could be calculated as:
where δ was the thickness of CMNR, mm.
The hydrogen production capacity was defined as the production of per membrane thickness of CMNR (Y H, mmol·h-1·mm-1) and it could be calculated as:
The gas permeability and the pressure drop of the CMNR was measured on an apparatus shown in Figure S5. And the permeability (m3·m-2·h-1·kPa-1) can be calculated as:
where F was the flowrate of nitrogen, m3·h-1; ΔP was the pressure drop of the transmembrane, kPa; A was the effective area of the CMNR, 3.14 cm-2.
The reactant gas flux through the membrane J(m3·m-2·h-1) can be calculated as:
where V was the volume of reactant gas, m3.