Multi-technical Approach Research Articles

Impact and efficiency of GH10 and GH11 thermostable endoxylanases on wheat bran and alkali-extractable arabinoxylans

The results of a comparative study of two thermostable (1→4)-β-xylan endoxylanases using a multi-technical approach indicate that a GH11 xylanase is more useful than a GH10 xylanase for the upgrading of wheat bran into soluble oligosaccharides. Both enzymes liberated complex mixtures of xylooligosaccharides. 13C NMR analysis provided evidence that xylanases cause the co-solubilisation of β-glucan, which is a result of cell-wall disassembly. The simultaneous use of both xylanases did not result in a synergistic action on wheat bran arabinoxylans, but instead led to the production of a product mixture whose profile resembled that produced by the action of the GH10 xylanase alone. Upon treatment with either xylanase, the diferulic acid levels in residual bran were unaltered, whereas content in ferulic and p-coumaric acids were unequally decreased. With regard to the major differences between the enzymes, the products resulting from the action of the GH10 xylanase were smaller in size than those produced by the GH11 xylanase, indicating a higher proportion of cleavage sites for the GH10 xylanase. The comparison of the kinetic parameters of each xylanase using various alkali-extractable arabinoxylans indicated that the GH10 xylanase was most active on soluble arabinoxylans. In contrast, probably because GH11 xylanase can better penetrate the cell-wall network, this enzyme was more efficient than the GH10 xylanase in the hydrolysis of wheat bran. Indeed the former enzyme displayed a nearly 2-fold higher affinity and a 6.8-fold higher turnover rate in the presence of this important by-product of the milling industry.

A comparative and multitechnical approach to the acid character of templated and non-templated ZSM-5 zeolites

ZSM-5 zeolites with varying SiO 2/Al 2O 3 ratios (28, 50, 75, 100, 150) were synthesized with and without the aid of a template. The molar oxide compositions of the reactants were xSiO 2:Al 2O 3: yNa 2O:1500 H 2O with x=28–150 and y=6.7. The samples were characterized by XRD, IR, SEM, and BET surface area measurements. The zeolites synthesized with template were found to be pure. Non-templated zeolites with SiO 2/Al 2O 3 greater than 50, however, contained α-quartz along with ZSM-5. The acidity was measured by stepwise temperature-programmed desorption (STPD) of ammonia and calorimetric heats of adsorption of ammonia. The number of acid sites decreased almost linearly with the SiO 2/Al 2O 3 ratio of the zeolites. It was found that the acidity of zeolites measured both by STPD of ammonia and calorimetry was in fairly good agreement. Aniline alkylation and cumene cracking reactions were carried out over the two series of zeolites to evaluate the catalytic properties involving acidity. Cumene-cracking was correlated with the number of strong acid sites in the zeolite as determined by microcalorimetry. In both series of zeolites, certainly Brönsted and Lewis acid sites were present. The possible presence of more Lewis sites in non-templated zeolites than in templated zeolites might be responsible for high aniline alkylation and low cumene-cracking activity.

Nature and strength of acid sites in HY zeolites: a multitechnical approach

The combination of several physico-chemical techniques such as temperature programmed desorption, microcalorimetry, infrared spectroscopy and photoelectron spectroscopy, together with structural data (by XRD and NMR), has allowed a detailed study of the acidity of commercial HY faujasites, both non-dealuminated and dealuminated by hydrothermal treatment. These techniques have been used with different basic probe molecules such as ammonia, pyridine and 2,6-lutidine (2,6-dimethylpyridine). They have demonstrated the existence of three different regions of acid strength. The weak sites associated to NH 3 adsorption heats between 70 and 130 kJ mol −1 are of Lewis and Brönsted types, the former being predominent in the dealuminated zeolites. The sites of intermediary strength are essentially the framework bridged hydroxyl groups (heats between 130 and 150 kJ mol −1). The strong sites (heats above 160 kJ mol −1) contain a significant number of extra-framework Lewis-type sites, but also Brönsted sites. The strength of these sites seems to be due to mutual interactions between Lewis and Brönsted sites. The use of 2,6-lutidine has furthermore allowed us to differentiate between two different populations of Lewis sites.