Transhydrogenation of pentane and 1-hexyne over CrOx/Al2O3 and potassium-doped CrOx/Al2O3 catalysts

Abstract

The transhydrogenation of pentane (P) and 1-hexyne (1HY) was investigated over 4% CrO x /Al 2 O 3 and potassium-doped 4% CrO x /Al 2 O 3 catalysts over a range of temperatures (523-773 K) with a 5:1 P:1HY ratio. Over the CrO x /Al 2 O 3 catalyst, transhydrogenation clearly occurred at temperatures below 625 K where the yield of alkenes was higher for the co-fed system than for a combination of the individual yields. Due to the acidic nature of the alumina, many of the products were alkylated olefins and alkylated hydrocarbons formed by coincident alkylation and isomerisation. When pentane was added to a feed containing 1-hexyne, the extent of carbon deposition was reduced. By comparing transhydrogenation to limited hydrogen 1-hexyne hydrogenation at 623 K, it was shown that the processes of hydrogenation and transhydrogenation were different, with hydrogenation favouring alkanes, while transhydrogenation favoured alkenes. This may be because pentane dehydrogenation only releases two hydrogen atoms, which only allows 1-hexyne to hydrogenate to 1-hexene. Therefore, if the rate of alkene isomerisation and desorption is faster than that of pentane dehydrogenation, only alkenes will be observed. The latter proposal would suggest that the dehydrogenation/hydrogenation process is closely coupled and would be consistent with pentane influencing 1-hexyne surface chemistry. The effect of the potassium doping was to increase the yield of alkenes. The reason for this may be related to changes in the nature of the surface chromia species. The potassium also neutralised the acid sites on the alumina, reducing the extent of alkylation and hydrogenolysis, which suppressed the formation of other alkynes in the product mix.

Transhydrogenation of pentane with 1,5- and 2,4-hexadiene over CrOx/Al2O3

“…The activity and selectivity of the catalyst were determined using an all-glass, atmospheric pressure, flow, microreactor. The block schematic of the flow reactor has been reported earlier [17]. The catalyst (0.5 g) was reduced in situ with pure hydrogen (40 ml min −1 ) by heating to 823 K and holding at this temperature for 2 h. After reduction was completed, the flow was switched to argon (40 ml min −1 ) and the system purged for 0.5 h. Pentane (99 + % Aldrich) and 1,5-hexadiene (97 + % Aldrich) or 2,4-hexadiene (97 + % Aldrich) were introduced to the argon carrier gas (30 ml min −1 ) using temperature-controlled vapourisers.…”

“…The reaction products were condensed and collected over a 2-h period in heptane at 253 K and then the cumulative volume was sampled and analysed by gas chromatography (Agilent series, FID detector) fitted with a 150-m Petrocol column. An online ESS mass spectrometer was also coupled to the reactor to continuously follow any non-condensed eluted products over the 2-h time period of the reactions [17]. The reaction was studied with each component individually and as a mixed feed, in the temperature range 523-773 K at atmospheric pressure.…”

“…It also can act as an acceptor for the continuous supply of hydrogen produced by alkane dehydrogenation. Transhydrogenation combines both processes as a single process [13] Recent literature has revealed transhydrogenation reactions on various catalysts such as supported vanadia, chromia and doped chromia, most of which involved the use of LPG hydrocarbons [7,[14][15][16][17], these research studies showed clearly that transhydrogenation occurs. Significant conversion and yield of the olefin are reported, for instance, Jackson et al reported ~ 72% increase in propene from propyne and butane transhydrogenation using 1% vanadia/θ-alumina catalyst at 823 K. Most of this research [7,[14][15][16] has been limited to C 2 -C 4 substrates.…”

“…Significant conversion and yield of the olefin are reported, for instance, Jackson et al reported ~ 72% increase in propene from propyne and butane transhydrogenation using 1% vanadia/θ-alumina catalyst at 823 K. Most of this research [7,[14][15][16] has been limited to C 2 -C 4 substrates. In our recent work [17], we extended this approach using a wider range of hydrocarbons (i.e., C-5 and C-6). Pentane and 1-hexyne transhydrogenation was investigated over chromia catalysts and revealed that transhydrogenation clearly occurred, especially at low temperatures [17].…”

“…In our recent work [17], we extended this approach using a wider range of hydrocarbons (i.e., C-5 and C-6). Pentane and 1-hexyne transhydrogenation was investigated over chromia catalysts and revealed that transhydrogenation clearly occurred, especially at low temperatures [17]. In this paper, the behaviour of conjugated and non-conjugated hexadiene isomers (2,4-hexadiene and 1,5-hexadiene respectively) for transhydrogenation with pentane was investigated using chromia catalyst.…”