Catalysis of Organic..

Liu, Cant and Smith 139an exact fit at the maximum but at the expense of worse fit at the extremes. Only asmall fraction of the apparent increase in rate at high conversion can be explained interms of an increase in the ratio of catalyst weight to liquid volume as liquid samplesare removed for analysis. Most of the discrepancy must arise from another cause.One possibility is that the difference in rate constant between DEA and HEG arises,at least in part, because they compete for catalyst surface with HEG beingdisfavoured when its mole fraction is low, perhaps because it lacks the second OH asan attachment point to the surface. In this case the reduced concentration of DEA athigh conversion would permit greater access by HEG leading to a faster end stagereaction.Table 4. Estimated values for k 1 and k 2 for the reaction of diethanolamineCu catalyst Cu-Cr catalystk 1 , L g -1 h -1 0.009 0.013k 2 , L g -1 h -1 0.004 0.009ConclusionsThe oxidative dehydrogenation of alcohols over skeletal copper catalysts exhibits awide range of rates. Inductive effects can account for some of the differences butother factors, possibly related to binding between nitrogen atoms and the surface, arealso present. Diethanolamine reacts sequentially with 2-hydroxyethylglycine as anintermediate. Changes in mole fraction, measured by 1 H NMR, during much of thereaction can be modeled according to sequential first order kinetics but the finalstages of reaction are faster than predicted.Experimental SectionUnpromoted and chromia-promoted skeletal copper catalysts were prepared asdescribed in detail previously (10, 11, 14, 15) by leaching a CuAl 2 alloy, sieved to106-211µm, in a large excess (500 mL) of 6.1 M NaOH, either alone or containingNa 2 CrO 4 (0.004 M), for 24 hours at 5°C.Each batch of catalyst was characterised as to BET surface area, Al content,Cr 2 O 3 content, and tested in the modified autoclave described earlier (10). Thiscomprised a 300 mL stainless steel autoclave (Parr Inst. Co., series 4560) modifiedwith a side valve connected to an independently heated stainless steel cylinder(150 mL). Each sample of catalyst (~9 g) was first pretreated in the autoclave in ~9.7M NaOH at 200°C for 6 hours. After cooling it was washed with distilled water anda second quantity of NaOH solution (identical to the above) added. The autoclavewas flushed with N 2 and pressurised to ~2 bar at room temperature. The temperaturewas brought to near the desired reaction temperature (160°C, ~6 bar) on a linearramp over a period of 45 minutes with the stirrer operating at 80 rpm. 60-70mL ofalcohol was preheated in the reservoir to a matching temperature and then introducedusing N 2 at >10 bar as a driving gas. The alcohols tested were obtained fromcommercial sources with minimum stated purities of at least 95% (97% or more inmost cases).