Isoprene synthesis from formaldehyde and isobutene over Keggin-type heteropolyacids supported on silica

Abstract

Isoprene synthesis from formaldehyde and isobutene over Keggin-type heteropolyacids supported on silica † V. L. Sushkevich, * a V. V. Ordomsky ab and I. I. Ivanova ac Gas phase Prins condensation of isobutene with formaldehyde has been studied over different Keggin-type heteropolyacids supported on amorphous silica. The catalysts were characterized by elemental analysis, X-ray diffraction, low temperature nitrogen adsorption, TPD of ammonia, FTIR of adsorbed pyridine and NMR spectroscopy. The activity of the supported heteropoly compounds was found to increase in the following order: H 4 SiMo 12 O 40 < H 3 PMo 12 O 40 < H 4 SiW 12 O 40 ≈ H 3 PW 12 O 40 . The lower activity of the supported molybdenum heteropolyacids was attributed to their low thermal stability and partial decomposition during catalyst activation, which resulted in lower acidity. The variation of HPA content from 5 to 33 wt% was also shown to increase catalyst activity. Based on the relationship between the content of weak Brønsted sites, the amount and type of carbonaceous deposits and the catalytic activity, it was concluded that the generation of "working" active sites over HPA catalysts involves the formation of unsaturated branched surface species over weak Brønsted sites. These active carbonaceous species are responsible for selective isoprene synthesis. The best catalyst performance is observed over the catalyst with 20 wt% of H 3 PW 12 O 40 , which shows an isoprene yield of 48% with a selectivity of 63%.Catal. Sci. Technol. This journal is Scheme 1 Two-step process for on-purpose isoprene synthesis from isobutene and formaldehyde.

Assembly line synthesis of isoprene from formaldehyde and isobutene over SiO2-supported MoP catalysts with active deposited carbon

“…The olenic carbons are most probably protonated at the acidic sites under high temperature to give working sites as C + . 9,21 Furthermore, the ratio of the two peaks changed slightly with TOS, which meant the coke components changed little, which was much different from this reaction over NbP catalysts. In the induction period, there was lower selectivity due to the insufficient number of working sites of C + because of the low carbon deposition.…”

“…16 The earlier route for IP synthesis was based on a two-step reaction: 17 (1) the condensation of IB and formaldehyde to produce 4,4dimethyldioxane-1,3 (DMD) in the liquid phase, and (2) the cracking of DMD into IP over solid catalysts (Scheme 1a). 18 The two-step process has various drawbacks, [19][20][21] and consequently a new promising route is urgently needed. Developing a onestep selective process is a very attractive pathway in this regard on account of its simplied procedures, and due to the possibility of easily manufacturing high purity isoprene (Scheme 1a).…”

“…Developing a onestep selective process is a very attractive pathway in this regard on account of its simplied procedures, and due to the possibility of easily manufacturing high purity isoprene (Scheme 1a). This process is based on heterogeneous catalysts, include zeolites, 22,23 phosphates, 20,24 sulfates, 25 oxide catalysts, 26 heteropolyacids, 19,21,27 and silver catalysts. 28 These reactions are associated with high initial yields of isoprene (60-70%), but also involve a rather low selectivity and a short catalyst life time due to its rapid deactivation.…”

“…Moreover, the main byproducts were investigated, it was found that carbon monoxide formation proceeded via formaldehyde decomposition over Lewis acid sites, DMD was observed during the steady-state period, and the main reaction network involving methylbutenols was also proposed. 21 However, this network was proposed without consideration of the inuence of the catalysts, and indeed, how the original sites and forming sites (carbon) work is still unknown, in which also the inuence of catalysts variation on the active sites and the chemical route is still unresolved.…”

Facile synthesis of hierarchically porous carbonaceous materials derived from olefin/aldehyde precursors using silica as templates

“…21,22 Very recently, it was uncovered that the reaction of isobutene and formaldehyde on acid sites leads to the formation of high amounts of carbonaceous deposits on the catalysts. 23 Sushkevich V. L. et al 24 reported a heterogeneous catalyst with 3D ordered carbonaceous framework through the Prins condensation over the solid acid catalysts, the important key of such method is heavily coke deposited on the catalyst in the course of the vapor reaction. Herein, inspired by huge carbonaceous deposition on the acid catalyst, we used phosphorus modied mesoporous silica as template and the impregnated acid could lead to the selective formation of carbon frameworks inside the template mesopores, preventing external carbon deposition.…”