Adsorption of pyridine (Py) on the surface of acid solids is frequently used for the characterization of the surface acidity. The use of IR spectroscopy to analyze adsorbed pyridine also allows discriminating between the different acid sites. The coordination of Py molecules to the surface is indicative of exposure on the surface of coordinatively unsaturated metal sites, i.e. Lewis acid sites. On the other hand, protonation of Py molecules into pyridinium ions are indicative of the availability of Brönsted acid sites (bridging or multicentered OH groups and coordinated H2O molecules) [45]. Occurrence of both acid sites is likely to happen in crystalline zirconia. Figure 5.9 represents the FTIR spectra of ZrNF-500 before and after pyridine chemisorption over the frequency range of ring breathing modes of vibration. The band at 1635 cm-1 in the ZrNF-500 spectrum is attributed to the flexion vibration of the O-H band of terminal hydroxyl groups. The presence of a band at 1620 cm-1 could be associated to the υ8a vibration mode of pyridine and suggests the presence of Lewis acid sites, which is supported by the peak at 1441 cm-1, related to the υ19b vibration mode of pyridine on Lewis acid sites [45]. On the other hand, appearance of protonated pyridine species over Brönsted acid sites could not be undoubtedly concluded. In our case, it is clearly seen a peak at 1540 cm-1, which is assessed to υ19b vibration mode of pyridine on
Brönsted acid sites; however, the υ8a vibration mode (1640–1630 cm−1) is not clearly resolved [45]. Nonetheless, the presence of –OH groups in terminal positions in the crystalline framework which could work as Brönsted acids was confirmed by XPS O1s and Zr3d spectra, along with Lewis acidity detected on surface, makes conceivable further use of these fibers as catalyst in methanol dehydration.
Figure 5.9. FTIR spectra of ZrNF-500 sample without and with chemisorbed pyridine
[ZrNF-500 (Py)]. Methanol decomposition
5.3.2.
5.3.2.1. Methanol decomposition over calcined zirconia nanofibers Figure 5.10.a represents steady state methanol conversions as a function of reaction temperature at an inlet methanol vapor pressure of 0.02 atm and space time of 0.214 g·s/µmol for zirconia fibers calcined at different temperatures, 500, 800 and 1000 ºC. ZrNF-500 shows a similar performance (conversion and selectivity) to that reported for grounded (20-42 mesh) pure zirconia catalyst pre-conditioned at 550 ºC in air for 3.5 hours, working at higher methanol partial pressure (PMeOH= 0.15 atm and space time of 0.077 g·s/µmol) [27]. The highest calcination temperatures analyzed, 1000 ºC, results in a catalyst with the lowest methanol steady state conversion values. Li et al. [46] observed changes in the selectivity of the CO hydrogenation reaction for monoclinic
and tetragonal zirconia and Stichert et al. [47] stated that the activity of monoclinic sulfate zirconia is lower by a factor of 2–5 compared to that of tetragonal sulfated zirconia for n-butane isomerization in a fixed bed reactor.
Differences in activity could be associated to the crystal size and surface properties of the fibers by changing from tetragonal to monoclinic phase, or to differences in surface acidity, i.e. nature, strength and amount of acid sites, which happens to be related to crystalline properties and calcination temperature. Brönsted acid sites or Lewis acid–base pair sites are believed to play a role in methanol dehydration and, generally, the stronger the acid sites the more active the catalysts. As far as Brönsted sites are involved, their strength and the reaction temperature should be controlled if hydrocarbons formation is to be avoided. The reaction mechanism based on Lewis acidity, on the other hand, requires an adjacent acid–base pair sites to provide the reaction between the adsorbed alcohol molecule on an acidic site and an adsorbed alkoxide anion on a basic site [48-50]. Fibers calcined at high temperatures have faced a more pronounced dehydroxilation, rendering lower number of Brönsted acid sites than ZrNF-500. For the fibers calcined at 1000 ºC, although in monoclinic zirconia high- temperature annealing favors the production of more oxygen defects, i.e. Lewis sites [44], the number of those acid sites available for methanol adsorption is lower, as sintering leads to higher crystal size which reduces the surface/bulk zirconia amount ratio.
Figure 5.10. Steady state methanol conversions (a) and distribution of carbon products
for ZrNF-500 (circles), ZrNF-800 (triangle), ZrNF-1000 (diamond) (b) as a function of reaction temperature. Inlet methanol vapor pressure of 0.04 atm, space time of 0.214
g·s·µmol-1.
Figure 5.10.b represents the distribution of carbon products as a function of reaction temperature for the zirconia nanofibers calcined at 500, 800 and 1000ºC. DME, CO, H2, H2O were the main products found by GC/MS at reactor exit. Propene and small amounts of C4 hydrocarbons, named olefins for brevity sake, were also found at low temperatures. Secondary products were methane and CO2 at T>500ºC, showing distribution values lower than 10%. H2O/(DME +olefin/2) molar ratio values were found to be close to 1, whereas H2/CO ratio values were near 2, which suggest that syngas is produced from methanol rather than for DME, being differences accountable to hydrogen formed in methanation of methanol. Small traces of ethane were found for ZrNF-500 at 550ºC. The low amount of ethene obtained in our study is in agreement with the previously reported by Gayubo et al [37] for the methanol-to-olefins process over a SAPO-18 catalyst. In general trend, at low-intermediate reaction temperatures methanol is mainly dehydrated to dimethyl ether and olefins (propylene or butenes), while at high reaction temperatures methanol begins to decompose to CO and hydrogen, with small participation of methanation [26,27,51]. Finally, the carbon dioxide could be produced by water-gas shift, the methane/CO2 ratio observed, close to a value of 1, suggests that it is probably formed as a side product of methanol methanation and perhaps in Boudart reaction (2CO C + CO2). Selectivity values to DME for the different catalysts followed the order ZrNF-500>ZrNF-800>ZrNF-1000. In consequence, the kinetics of methanol dehydration was studied in ZrNF-500; although the ones calcined at 800 and 1000 ºC render high selectivity to syngas, which could be of interest for catalyst support in hydrogen production from methanol.