The catalytic activity on the coprecipitated Cu–ZnO–Al2O3/Zr-ferrierite (CZA–ZrFER) with different Zr content from 0 to 5 wt.% was investigated for the direct synthesis of dimethylether (DME) from H2- deficient and biomass-derived model syngas (H2/CO molar ratio = 0.93). The catalytic functionalities, such as CO conversion and DME selectivity, showed their maxima on the bifunctional catalyst with 3 wt.% Zr-modified ferrierite. Detailed characterization studies were conducted on the catalysts to measure their properties such as surface area, acidity by temperature-programmed desorption of ammonia (NH3-TPD), reducibility of Cu oxide by temperature-programmed reduction (TPR), copper surface area measurements by N2O titration method, electronic states of copper by IR analysis and particle size measurement by XRD and TEM analysis. The number of acid sites measured by NH3-TPD on the bifunctional catalysts decreased monotonously with the increase of Zr content, meanwhile, the acidic strength is found to be minimal on the catalyst showing best performance. The reducibility of copper oxide and the surface area of metallic copper also exhibited their maximum values at the same Zr composition indicating that these are responsible for the optimum functionality of the bifunctional CZA– ZrFER catalyst. The role of easily reducible copper species with small particle size and the suppressed strong acidic sites is also emphasized in the consecutive reaction from syngas to DME on the bifunctional catalyst. The different behavior of intrinsic rate of the bifunctional catalysts is also well correlated with the metallic surface area of copper and the amount of acidic sites with their acidic strength.
Dimethylether (DME) is one of the important chemicals for the production of dimethyl sulfate, methyl acetate and light olefins and it is also considered to be an environmentally benign alternative clean fuel since it is easily handled and transported by using the infrastructure of LPG transportation due to its similar physical properties . In addition, as the oil resources are expected to deplete in near future. Alternative fuels such as a biomass-derived DME and methanol have been getting greater attention to minimize the emissions of global warming gases and hazardous components such as SOx, NOx and particulate matter. Furthermore, DME could be one of the possible candidates to supply hydrogen-rich fuel cell feed gas by autothermal reforming or steam reforming process . The general process for the production of DME from syngas consists of the synthesis of methanol by hydrogenation of CO and/or CO2 on Cu–ZnO-based catalysts followed by the production of DME by dehydration of MeOH on a solid acid catalyst like g-Al2O3 or modified ZSM-5 zeolite [3,4]. However, a lot of emphasis has been laid on the development of a single-step process for the direct synthesis of DME in a fixed-bed or slurry reactor  using bifunctional catalysts that exhibit the above two functionalities. A single-step reaction (syngas to DME; STD) of biomass-derived H2-deficient and CO2- abundant syngas  has gained tremendous attention for the synthesis of DME, due to its ability to produce renewable fuel with high potential to minimize the pollutant emissions into the atmosphere and thereby to reduce global warming. Preparation of bifunctional catalysts is a challenging process and it needs special efforts to balance the two functionalities (active sites for CO hydrogenation and dehydration of methanol). There are a few methods to achieve highly active and selective bifunctional catalyst. For example, the preparation of highly dispersed fine crystallites of skeletal Cu with high surface area can be achieved by changing the preparation conditions and eventual stabilization of active Cu species by adding promoters [7,8]; modification of acidity of bifunctional catalyst by modifying ZSM-5 with Fe  or Mg  and changing the silica-to-alumina ratio of amorphous silica–alumina catalyst . The modification of acidity is a key variable to enhance the DME selectivity and to suppress further dehydration of DME into undesired hydrocarbons. In our previous investigation, a superior catalytic performance was demonstrated on Zr-modified ferrierite bifunctional catalyst in terms of high CO conversion and DME selectivity compared to the ZSM-5 and Y zeolite-based bifunctional catalysts, owing to the facile reducibility of metal component and the presence of proper amount of acid sites that existed on the former . The superiority of ferrierite also lies in its favorable topology facilitating easy diffusion of the reactants and products . Although much work is reported on the role of acidity of the solid acid component, the importance of Zr composition in the bifunctional catalyst containing Cu–ZnO–Al2O3 and Zr-modified ferrierite is not investigated for the direct synthesis of DME from biomass-derived syngas.
Thus, the present investigation is focused on understanding the influence of Zr content on Zr-modified ferrierite which is introduced during the coprecipitation of Cu–ZnO–Al2O3 components to make a bifunctional catalyst on the CO conversion and DME selectivity to utilize biomass-derived syngas efficiently. The characterization of the bifunctional catalyst using various methods such as nitrogen adsorption, X-ray diffraction (XRD) analysis, the measurement of metallic copper surface area, temperatureprogrammed desorption of ammonia (NH3-TPD), temperatureprogrammed reduction (TPR), X-ray photoelectron spectroscopy (XPS), Fourier-transformed infrared (FT-IR) spectroscopy and transmission electron microscope (TEM) are utilized to study the effect of Zr content on the bifunctional Cu–ZnO–Al2O3/Zrmodified ferrierite catalyst for STD reaction.
2.1. Preparation of bifunctional catalysts
The Cu–ZnO–Al2O3/Zr-ferrierite bifunctional catalyst (CZA/ ZrFER) consisting of 7:3 ratio of Cu–ZnO–Al2O3 component (denoted as CZA, active components for methanol synthesis) and Zr-ferrierite component (denoted as ZrFER, active one for DME synthesis) with different Zr content was prepared by coprecipitation method in the slurry of ZrFER. The H-ferrierite has a surface area of 364 m2 /g and a pore volume of 0.13 cm3 /g. For the bifunctional CZA/ZrFER catalyst, a metal oxide composition was 50 wt.% CuO, 40 wt.% ZnO, and 10 wt.% Al2O3. In order to prepare the metal oxides the metal nitrates were selected as their precursors. The weight ratio of CZA component to ZrFER was further verified by XRF analysis. The ZrFER was prepared by loading Zr precursor (ZrCl2O8H2O) on the ferrierite (Si/Al = 25 provided by Zeolyst) with different Zr contents from 0 to 5 wt.% by slurry impregnation method. The ZrFER was previously calcined at 400 8C for 5 h under flowing air before making a bifunctional CZA– ZrFER catalyst. The bifunctional CZA–ZrFER catalyst was prepared by coprecipitation of CZA component in the slurry of ZrFER by using the Na2CO3 as a precipitant at a pH of around 7 and digested at 70 8C for 3 h. The finished catalyst was dried and subsequently calcined at 350 8C for 5 h in flowing air. A more detailed preparation method was reported in our previous work [12–14]. The catalysts are denoted as CZA–ZrFER(X), where CZA stands for the Cu–ZnO–Al2O3 component and ZrFER for the Zr-modified ferrierite. The X in the bracket denotes the Zr content such as 0, 1, 3 and 5 wt.% based on the 100 wt.% of bare ferrierite zeolite.
2.2. Catalyst characterization
The surface area and pore volume measurements were conducted by nitrogen adsorption and desorption isotherms obtained at 196 8C using a constant-volume adsorption apparatus (Micromeritics, ASAP-2400). The pore volume was determined at a relative pressure (P/Po) of 0.99. The calcined samples were degassed at 250 8C with a He flow for 4 h before the measurements. The pore size distribution of samples was determined by the BJH (Barett–Joyner–Halenda) model from the desorption branch of the nitrogen isotherm.
The fresh and used bifunctional CZA–ZrFER catalysts were thoroughly characterized for their structural identification and crystallinity by the powder X-ray diffraction (XRD) patterns (Rigaku diffractometer using Cu Ka radiation).
The Cu metal area was measured by N2O surface titration method by using the home-made apparatus. Prior to the titration, the fresh and used samples with 0.5 g were reduced at 250 8C for 4 h with 5% H2/N2 flow, followed by purging and cooling with a He flow to 100 8C. The consumption of N2O as well as the evolution of N2 on the metallic Cu sites (N2O + 2Cu = Cu2O+N2) was measured at 60 8C by a thermal conductivity detector (TCD). The surface area of metallic Cu was calculated by assuming 1.46 1019 Cu atoms/ m2 and a N2O/Cus (Cu atom on surface) molar stoichiometry of 0.5 and the particle size of Cu was calculated with the equation of 6000/(8.92 Cu metal surface area/Cu fraction in gram catalyst) .
The NH3-TPD experiments (Micromeritics, Autochem 2920) were performed to determine the acidity of calcined and used catalysts by using GC equipped with TCD. About 0.1 g of the sample was flushed initially with a He flow at 250 8C for 2 h, cooled to 100 8C and saturated with NH3. Purging the sample with He flow was continued until equilibrium, and then TPD was carried out from 100 to 500 8C at a heating rate of 10 8C/min and kept at that temperature for 20 min.
For the temperature-programmed reduction (TPR) carried out on the BEL-CAT instrument, the sample (0.1 g) was previously treated in He flow up to 350 8C and kept for 2 h to remove adsorbed water and other contaminants followed by cooling to 50 8C. The 5% H2/He mixture was passed over the samples at a flow rate of 30 ml/ min with a heating rate of 10 8C/min up to 450 8C. The effluent gas was passed over a molecular sieve trap to remove the generated water and then analyzed by GC equipped with TCD.