Optimization of the Technological Parameters for Obtaining Zn-Ti Based Composites to Increase the Performance of H 2 S Removal from Syngas

: The realization of some composite materials that allow the best removal of H 2 S from syngas was the main objective of this work. Thus, the optimization of the technological parameters for obtaining composites based on Zn-Ti was achieved. The paper studies the inﬂuence of calcination temperature on the characteristics of the binary ZnO-TiO 2 system used to synthesize a composite material with suitable properties to be used subsequently for syngas treatment. The mineralogical and structural analyzes showed that starting with the calcination temperature of 700 ◦ C the material synthetized is composed mainly of zinc orthotitanate which possess the corresponding characteristics to be ﬁnally used in the treatment of the syngas for its desulfurization. At this calcination temperature the material has a compact structure most likely due to sintering of the formed titanates. These composites have a texture that places them rather in the category of non-porous materials, the pore volume and their surface area obviously decreasing as the calcination temperature increases. A maximum sulfur removal degree of about 97% was obtained by using a composite synthetized at a temperature of 700 ◦ C (ZT-700).


Introduction
Gasification of renewable carbonaceous resources is currently one of the most important worldwide energy resources, mainly due to their availability, low cost, and environmental benefits [1,2]. Carbonaceous biomass such as sugar and starch crops (i.e., sugar beet, grains and tubers), oil crops (i.e., palm, rapeseed, sunflower), lignocellulosic plants (i.e., willow and eucalyptus), lignocellulosic biomass residue-derived agroforestry industries, and algae biomass, represents over 70% of all renewable energy production, and up to 10% of the worldwide total energy supply [3]. Steam gasification or supercritical water gasification in conjunction with wide range of catalysts such as alkaline earth metallic

Composites Synthesis
The ZnO-TiO 2 composites were synthetized through a simple precipitation method which is described in the following. In this respect, ZnO and TiO 2 powders were initially dry mixed at a ZnO:TiO 2 molar ratio of 2:1 for 30 min, after which a solution of ammonium bicarbonate (12.5% NH 4 HCO 3 ) was gradually added for 60 min until a consistent paste was obtained. Next, the paste obtained was vacuum oven dried at 105 • C until a constant mass was reached. The dried paste was pre-calcined at 300 • C for four hours. The obtained product was divided into five samples that were subjected to calcination for four hours at different temperatures. The synthesis conditions are presented in Table 1.

Composites Characterization
The phase composition analysis of the composites was performed by X-ray diffractometry (XRD) using a model 6000 diffractometer (Shimadzu, Duisburg, Germany; −2θ Bragg-Brentano geometry, using the CuKα characteristic radiations). The elimination of the CuKβ component was achieved by a Ni filter. The experimental data were digitally collected through "step by step" scanning method in the 2θ angle interval of 10-90 degrees. Scanning Electron Microscopy (SEM) was performed by using a S2600N scanning electron microscope (Hitachi, Berkshire, United Kingdom). The X-ray qualitative and quantitative microanalyses were performed with an X-ray spectra energy dispersion of Röntec type Brunauer-Emmett-Teller (BET) type. The textural analysis was performed by using an ASAP 2020 physisorption analyzer (Micromeritics" Unterschleissheim, Germany). The samples were first degassed for 2 h at a temperature of 150 • C and pressure of 0.1 Pa and then subjected to analysis. The N 2 adsorption-desorption isotherms of composites were determined at N 2 liquefaction temperature (77.35 K). The surface area was determined from the isotherms data by using Brunauer-Emmett-Teller (BET) method, while the pore size distribution and the pore volume were calculated by using Barrett-Joyner-Halenda (BJH) method. FTIR spectra of composites were recorded with a Tensor 37 instrument (Bruker, Durham, United Kingdom), in attenuated total reflectance mode (ATR, Golden Gate diamond unit). The wavelength range was from 4000 to 400 cm −1 at 64 scans per spectrum, with a resolution of 4 cm −1 .

ZnO-TiO 2 Composite Sulfurization
Two grams of ZnO-TiO 2 composite with a particle size less than 1 mm were sulfurized for five hours by passing a H 2 S gas stream through a tubular reactor heated at a temperature of 500 ± 2 • C with a flow rate of 50 mL/min. H 2 S was generated by the acid attack (300 mL hydrochloric acid-HCl, 30-35% by mass) of 50 g of iron sulfide (FeS). Prior to sulfurization, the ZnO-TiO 2 composite was activated by passing an argon stream through the tubular reactor heated to 300 • C for 20 min. A sketch of the sulfurization laboratory plant is shown in Figure 1. The installation used for sulfurization is not a standard one. It was developed in-house by the authors.

ZnO-TiO2 Composite Regeneration
The regeneration of the sulfurized ZnO-TiO2 composite was carried out by combustion the samples into an oxygen-excess atmosphere. In this respect, 0.2 g of sulfurized ZnO-TiO2 composite with a particle size less than 1 mm were introduced into an electrically heated oven, preheated at a temperature of 400 °C, which is connected to the gas capture and analysis facility of the formed sulfur dioxide (SO2). The heating of the oven continues up to a temperature of 950 °C with a rate of 10 °C/min. The sulfur dioxide formed is trapped in an iodine solution (I2, 0.1 N) added progressively to the bubbling vessel until it has not decolorized anymore. The excess iodine is subsequently titrated with sodium thiosulfate solution (Na2S2O3, 0.1 N) in the presence of starch solution. A sketch of the laboratory plant is shown in Figure 2. The method and the installation used for composite regeneration are not standard. Both of them were developed by us.

ZnO-TiO 2 Composite Regeneration
The regeneration of the sulfurized ZnO-TiO 2 composite was carried out by combustion the samples into an oxygen-excess atmosphere. In this respect, 0.2 g of sulfurized ZnO-TiO 2 composite with a particle size less than 1 mm were introduced into an electrically heated oven, preheated at a temperature of 400 • C, which is connected to the gas capture and analysis facility of the formed sulfur dioxide (SO 2 ). The heating of the oven continues up to a temperature of 950 • C with a rate of 10 • C/min. The sulfur dioxide formed is trapped in an iodine solution (I 2 , 0.1 N) added progressively to the bubbling vessel until it has not decolorized anymore. The excess iodine is subsequently titrated with sodium thiosulfate solution (Na 2 S 2 O 3 , 0.1 N) in the presence of starch solution. A sketch of the laboratory plant is shown in Figure 2. The method and the installation used for composite regeneration are not standard. Both of them were developed by us.

ZnO-TiO2 Composite Regeneration
The regeneration of the sulfurized ZnO-TiO2 composite was carried out by combustion the samples into an oxygen-excess atmosphere. In this respect, 0.2 g of sulfurized ZnO-TiO2 composite with a particle size less than 1 mm were introduced into an electrically heated oven, preheated at a temperature of 400 °C, which is connected to the gas capture and analysis facility of the formed sulfur dioxide (SO2). The heating of the oven continues up to a temperature of 950 °C with a rate of 10 °C/min. The sulfur dioxide formed is trapped in an iodine solution (I2, 0.1 N) added progressively to the bubbling vessel until it has not decolorized anymore. The excess iodine is subsequently titrated with sodium thiosulfate solution (Na2S2O3, 0.1 N) in the presence of starch solution. A sketch of the laboratory plant is shown in Figure 2. The method and the installation used for composite regeneration are not standard. Both of them were developed by us.

Infrared Spectroscopy
The FTIR spectra of the synthetized composites are presented in Figure 4. As can be seen, the spectra corresponding to the composites ZT-300, ZT-500, and ZT-600 show a broad absorption band in the range of 3371 to 3381 cm −1 , which was attributed to the O-H stretching vibration of the water molecules. This absorption band disappears in the spectra of the composites heat-treated at high temperatures, namely ZT-700 and ZT-800, which is an indicator for the complete dehydration of the ZnO-TiO2 binary system. The absorption bands in the frequency intervals of 2318 to 2353 cm −1 was associated with the free CO2 molecule existing in the atmospheric air [20]. Regarding ZT-800, the weight percent of Zn 2 TiO 4 is 88%, and only 1.8% TiO 2 (rutile form) and 9.8% ZnO remains unreacted. The difference regarding the participation in the reaction of the two oxides components of the ZnO-TiO 2 binary system is that at high calcination temperatures titanium ions (Ti 4+ ) and zinc ions (Zn 2+ ) diffuse at different rates through the zinc oxide layer, the rate of diffusion of titanium ions being greater than that of zinc ions [22,23].

Infrared Spectroscopy
The FTIR spectra of the synthetized composites are presented in Figure 4. As can be seen, the spectra corresponding to the composites ZT-300, ZT-500, and ZT-600 show a broad absorption band in the range of 3371 to 3381 cm −1 , which was attributed to the O-H stretching vibration of the water molecules. This absorption band disappears in the spectra of the composites heat-treated at high temperatures, namely ZT-700 and ZT-800, which is an indicator for the complete dehydration of the ZnO-TiO 2 binary system. The absorption bands in the frequency intervals of 2318 to 2353 cm −1 was associated with the free CO 2 molecule existing in the atmospheric air [20].
Also, the absorption bands (spectra of ZT-300, ZT-500, and ZT-600) in the range of 1409-1411 cm −1 and those in the range of 1501-1504 cm −1 have been associated with carbonate species which could be derived from the zinc titanate precursors. The peaks located in the range of 722-797 cm −1 in the spectra of ZT-300, ZT-500, ZT-600, and ZT-700 are assigned to Ti-O stretching vibration in the octahedral TiO 6 group which is present in TiO 2 , ZnTiO 3 and Zn 2 TiO 4 [16,17,24]. It should be noted that, of all these spectra, the one corresponding to the ZT-700 has an adsorption band in this frequency interval with the lowest intensity. However, unlike the others, it presents a new absorption band located at 549 cm −1 which was assigned to the same Ti-O stretching vibration which is this time associated with the octahedral TiO 6 group in the Zn 2 TiO 4 . The spectrum of ZT-800 has a single absorption band located at approximately the same frequency as in the case of ZT-700 (548 cm −1 ) spectrum, which is also associated with TiO 6 group in the Zn 2 TiO 4 . The peak that appears at 485 cm −1 in the spectra of ZT-300, ZT-500, and ZT-600 was assigned to the stretching vibrations of Zn-O bond in ZnO. All these results indicate a progressive transformation of the ZnO-TiO 2 binary system with the increase of the calcination temperature, which becomes predominantly mono-component (i.e., Zn 2 TiO 4 ) at calcination temperatures above 700 • C. These results are consistent with those obtained by XRD analysis. ZnO-TiO2 binary system. The absorption bands in the frequency intervals of 2318 to 2353 cm −1 was associated with the free CO2 molecule existing in the atmospheric air [20]. Also, the absorption bands (spectra of ZT-300, ZT-500, and ZT-600) in the range of 1409-1411 cm −1 and those in the range of 1501-1504 cm −1 have been associated with carbonate species which could be derived from the zinc titanate precursors. The peaks located in the range of 722-797 cm −1 in the spectra of ZT-300, ZT-500, ZT-600, and ZT-700 are assigned to Ti-O stretching vibration in the octahedral TiO6 group which is present in TiO2, ZnTiO3 and Zn2TiO4 [16,17,24]. It should be noted that, of all these spectra, the one corresponding to the ZT-700 has an adsorption band in this frequency interval with the lowest intensity. However, unlike the others, it presents a new absorption band located at 549 cm −1 which was assigned to the same Ti-O stretching vibration which is this time associated with the octahedral TiO6 group in the Zn2TiO4. The spectrum of ZT-800 has a single absorption band located at approximately the same frequency as in the case of ZT-700 (548 cm −1 ) spectrum, which is also associated with TiO6 group in the Zn2TiO4. The peak that appears at 485 cm -1 in the spectra of ZT-300, ZT-500, and ZT-600 was assigned to the stretching vibrations of Zn-O bond in ZnO. All these results indicate a progressive transformation of the ZnO-TiO2 binary system with the increase of the calcination temperature, which becomes predominantly mono-component (i.e., Zn2TiO4) at calcination temperatures above 700 °C. These results are consistent with those obtained by XRD analysis.    It is also noted that some particles have brighter edges, which suggests their orientation at different heights. The electron micrograph corresponding to the ZT-500 is similar to that of ZT-300 showing large ZnO particles with dimensions ranging between 119-200 nm, TiO2 particles in anatase form, whose dimensions are small, ranging between 15-22 nm, as well as intermediate-size particles of TiO2 in the rutile form with dimensions ranged between 80-100 nm. The morphology of ZT-600 begins to look uniform with the dimensions of particles ranging between 18-105 nm. At this calcination temperature, TiO2 particles in the anatase form get polygonal shapes in comparison with ZT-500 in which the TiO2 particles do not have a defined form. ZT-700 has a uniform morphology with particles having similar dimensions and regular forms. This morphology suggests the conversion of a large part of TiO2 and ZnO to zinc titanates, as was also confirmed by XRD results. The particles have a clear outline being in the form of plates stacked one above another with the dimensions ranging between 100-350 nm. Over the calcination temperature of 700 °C zinc titanates particles get sintered and the contours of the particles begin to disappear (electron micrograph of ZT-800). The dimension of particles varies between 110-177 nm.

Textural Characterization of Synthetized Composites
To characterize the synthesized composites from a textural point of view, a BET analysis of their surfaces was carried out. The adsorption-desorption isotherms as well as the particle size distribution of the synthetized composites are presented in Figure 6.
According to the IUPAC classification [25], the measured adsorption-desorption isotherms of ZT-500 and ZT-600 are of type II (the relative pressure-p/p0 at which the multilayer adsorption begins is approximately 0.2 for ZT-500 and 0.1 for ZT-600) which indicates that these composites have developed a predominantly non-porous structure with only a limited number of micropores and mesopores.
However, the small type H3 hysteresis that appears on these isotherms indicates some plate-like particles with irregular and slit-shaped pores. The measured adsorption-desorption isotherms of ZT-700 looks rather to be of type III which is also associated with a non-porous material. However, the open hysteresis that appears on the isotherm suggests a small micropore volume developed in this It is also noted that some particles have brighter edges, which suggests their orientation at different heights. The electron micrograph corresponding to the ZT-500 is similar to that of ZT-300 showing large ZnO particles with dimensions ranging between 119-200 nm, TiO 2 particles in anatase form, whose dimensions are small, ranging between 15-22 nm, as well as intermediate-size particles of TiO 2 in the rutile form with dimensions ranged between 80-100 nm. The morphology of ZT-600 begins to look uniform with the dimensions of particles ranging between 18-105 nm. At this calcination temperature, TiO 2 particles in the anatase form get polygonal shapes in comparison with ZT-500 in which the TiO 2 particles do not have a defined form. ZT-700 has a uniform morphology with particles having similar dimensions and regular forms. This morphology suggests the conversion of a large part of TiO 2 and ZnO to zinc titanates, as was also confirmed by XRD results. The particles have a clear outline being in the form of plates stacked one above another with the dimensions ranging between 100-350 nm. Over the calcination temperature of 700 • C zinc titanates particles get sintered and the contours of the particles begin to disappear (electron micrograph of ZT-800). The dimension of particles varies between 110-177 nm.

Textural Characterization of Synthetized Composites
To characterize the synthesized composites from a textural point of view, a BET analysis of their surfaces was carried out. The adsorption-desorption isotherms as well as the particle size distribution of the synthetized composites are presented in Figure 6.
According to the IUPAC classification [25], the measured adsorption-desorption isotherms of ZT-500 and ZT-600 are of type II (the relative pressure-p/p 0 at which the multilayer adsorption begins is approximately 0.2 for ZT-500 and 0.1 for ZT-600) which indicates that these composites have developed a predominantly non-porous structure with only a limited number of micropores and mesopores.
However, the small type H3 hysteresis that appears on these isotherms indicates some plate-like particles with irregular and slit-shaped pores. The measured adsorption-desorption isotherms of ZT-700 looks rather to be of type III which is also associated with a non-porous material. However, the open hysteresis that appears on the isotherm suggests a small micropore volume developed in this type of   Figure 7 shows the variation of BET surface area and pore volume of the synthesized composites according to the calcination temperature. As can be seen, both BET surface area and pore volume decrease with the increasing of calcination temperature. This is most probably due to the increasingly pronounced sintering of the formed zinc titanates, which leads to the closure of the pores [26]. According to the results obtained from the BET analysis, the largest surface area was registered for ZT-500, this being 11.97 m 2 /g, but as the XRD results have shown, at this calcination temperature the material consists only of Ti and Zn oxides. With the occurrence of zinc titanates in the system starting with the calcination temperature of 600 °C, the specific surface begins to decrease, so that for ZT-600  Figure 7 shows the variation of BET surface area and pore volume of the synthesized composites according to the calcination temperature. As can be seen, both BET surface area and pore volume decrease with the increasing of calcination temperature. This is most probably due to the increasingly pronounced sintering of the formed zinc titanates, which leads to the closure of the pores [26]. According to the results obtained from the BET analysis, the largest surface area was registered for ZT-500, this being 11.97 m 2 /g, but as the XRD results have shown, at this calcination temperature the material consists only of Ti and Zn oxides. With the occurrence of zinc titanates in the system starting with the calcination temperature of 600 • C, the specific surface begins to decrease, so that for ZT-600 this is 10.35 m 2 /g and for ZT-700 it is only 4.58 m 2 /g. Similar trend was recorded for pore volume of synthetized composites namely, 0.031 cm 3 /g for ZT-500, 0.015 cm 3 /g for ZT-600, and 0.014 cm 3 /g for ZT-700. This very small pore volume suggests a non-porous-like structure of the synthetized composites [27,28]. ZT-500, this being 11.97 m 2 /g, but as the XRD results have shown, at this calcination temperature the material consists only of Ti and Zn oxides. With the occurrence of zinc titanates in the system starting with the calcination temperature of 600 °C, the specific surface begins to decrease, so that for ZT-600 this is 10.35 m 2 /g and for ZT-700 it is only 4.58 m 2 /g. Similar trend was recorded for pore volume of synthetized composites namely, 0.031 cm 3 /g for ZT-500, 0.015 cm 3 /g for ZT-600, and 0.014 cm 3 /g for ZT-700. This very small pore volume suggests a non-porous-like structure of the synthetized composites [27,28].

ZnO-TiO2 Composite Testing
From the mineralogical, morphological, structural and textural analyzes it can be concluded that ZnO-TiO2 composite obtained by calcination at 700 °C (ZT-700) exhibits the best physicochemical characteristics and, therefore has been tested in respect with its capacity to remove H2S from a simulated gas stream. The removal degree of sulfur from simulated gas stream obtained with ZT-700 was established from the tests results. The obtained results are presented in Table 2 and Figure 8.

ZnO-TiO 2 Composite Testing
From the mineralogical, morphological, structural and textural analyzes it can be concluded that ZnO-TiO 2 composite obtained by calcination at 700 • C (ZT-700) exhibits the best physicochemical characteristics and, therefore has been tested in respect with its capacity to remove H 2 S from a simulated gas stream. The removal degree of sulfur from simulated gas stream obtained with ZT-700 was established from the tests results. The obtained results are presented in Table 2 and Figure 8. ZT-500, this being 11.97 m 2 /g, but as the XRD results have shown, at this calcination temperature the material consists only of Ti and Zn oxides. With the occurrence of zinc titanates in the system starting with the calcination temperature of 600 °C, the specific surface begins to decrease, so that for ZT-600 this is 10.35 m 2 /g and for ZT-700 it is only 4.58 m 2 /g. Similar trend was recorded for pore volume of synthetized composites namely, 0.031 cm 3 /g for ZT-500, 0.015 cm 3 /g for ZT-600, and 0.014 cm 3 /g for ZT-700. This very small pore volume suggests a non-porous-like structure of the synthetized composites [27,28].

ZnO-TiO2 Composite Testing
From the mineralogical, morphological, structural and textural analyzes it can be concluded that ZnO-TiO2 composite obtained by calcination at 700 °C (ZT-700) exhibits the best physicochemical characteristics and, therefore has been tested in respect with its capacity to remove H2S from a simulated gas stream. The removal degree of sulfur from simulated gas stream obtained with ZT-700 was established from the tests results. The obtained results are presented in Table 2 and Figure 8. * The theoretical amount of sulfur is determined by the molar ratio of oxides (ZnO:TiO2 = 2:1) relative to 100 g of sulfur-saturated mass of sample.  The equations of the chemical reactions as well as the calculations underlying the experimental determination of the experimental sulfur content in the sample are presented in Equations (1)- (6): The degree of sulfur removal obtained with ZT-700 was determined with Equation (7): where S e is the experimental sulfur content, % (wt.); S t is the theoretical sulfur content, % (wt.), η is the sulfur removal degree, %; V 1 is the volume of iodine consumed up to the total regeneration of the sulfurized ZT-700, mL; V 2 is the volume of 0.1 N sodium thiosulfate solution used for titration, mL; f 1 is the correction factor of the 0.1 N iodine solution; f 2 is the correction factor of the 0.1 N iodine solution sodium thiosulfate solution; m is the mass of the sulfurized ZT-700 sample, g; 0.0016 is the amount of sulfur (g) corresponding to 1 cm 3 of 0.1 N iodine solution.
As it shown in Figure 8, the oxidation of zinc sulfide (ZnS) begins at low temperature values (400-500 • C), and the maximum oxidation rate is reached in the temperature range of 600-700 • C. At these temperatures a content of sulfur of approximate 23% wt. is reached, which corresponds to a maximum sulfur removal degree of approximate 97%. Table 3 presents a comparative study on the efficiency of H 2 S removal from syngas between the results obtained in this work and those presented in the literature.

Conclusions
The influence of calcination temperature on the characteristics of the ZnO-TiO 2 composites was studied in this work through mineralogical, morphological, structural and textural analysis. From the phase composition standpoint, the results derived from both mineralogical and structural analyzes highlighted that with the increase of calcination temperature the ZnO-TiO 2 system evolves from an exclusively oxide system (calcination temperature of 500 • C) to a composite system consisting mainly of zinc orthotitanate (calcination temperature of 700 • C). The morphological analysis of the synthesized composites showed that as the calcination temperature increases, their component particles undergo a series of dimensional and shape changes that are closely related to the crystalline transformations of the oxide system and to the newly formed zinc titanate types. At high calcination temperatures (700-800 • C) the zinc titanates formed sintering leading to a compact structure of the synthesized composites. The textural analysis revealed the formation of a predominantly non-porous composites, the pore volume and their surface area decreasing with the increasing of calcination temperature.
Therefore, it was found that ZT-700 type composites, which contain predominantly zinc orthotitanate, meets the characteristics that recommend it to be successfully used for the desulfurization of the syngas. In this regard, the tests carried out to establish the sulfur removal capacity of ZT-700 from a simulated gas stream showed that a removal degree of about 97% can be reached, which open the way for further experiments with real syngas. Thus, given the need for power generation using advanced technologies, such as gas/turbine engines or solid fuel cells, it is necessary to reduce the H 2 S content to acceptable levels, and the use of ZT-700 type composites is a solution to achieve this objective. Also, in future research, will be pursued a better optimization of ZnO:TiO 2 molar ratio so as to increase the operating performance of these composites and to analyze the possibility of elimination of H 2 S and other types of gases.