Amorphous SiO2 catalyst for vapor-phase aldol condensation of butanal
Graphical abstract
Introduction
In the chemical industry, aldol condensation is an important reaction for the synthesis of long-chain chemicals [1,2], and it has wide applications in the field of biomass conversion to useful chemicals and fuels [[3], [4], [5]] as well as in the field of petrochemistry. Aldol condensation proceeds between aldehydes and/or ketones, and either an acid or a base can catalyze the reaction [1,2]. Industrially, sulfuric acid or sodium hydroxide is generally used as a catalyst in aldol condensation [1,2,[6], [7], [8], [9]], while it is obvious that the utilization of inorganic acid and base is not friendly to the environmental. The development of solid catalysts for aldol condensation has attracted much attention in recent years. Solid acids such as zeolites and heteropolyacids [7,[10], [11], [12], [13], [14]] as well as solid bases such as Na/SiO2 and MgO [[15], [16], [17], [18]] exhibit catalytic activity for aldol condensation, whereas the catalysts are not stable and they deactivated during the reaction due to a poisoning of the active sites by the deposition of carbonaceous species in most cases.
Self-aldol condensation of butanal to form 2-ethyl-2-hexenal (2E2H) is investigated as a model reaction to study aldol condensation reactions [14,[19], [20], [21], [22], [23], [24], [25]]. In addition, the reaction is attracting great importance because 2E2H has been used as an intermediate for producing perfumes, cosmetics, and plasticizers [19,20,23,24]. A vapor-phase continuous process for 2E2H production is attractive from the viewpoint of industrial application. However, serious deactivation of a catalyst is reported in the previous studies performed in a vapor phase [[20], [21], [22],25]. For example, the conversion of butanal over Pd/Na/SiO2 decreased from 81.8 to 49.1% with time on stream from 4 to 8 h at 350 °C [21]. In our recent report, SiO2-Al2O3, Nb2O5, and TiO2 were tested at 200 °C in H2 flow: all the catalysts gradually deactivated and the conversion of butanal decreased with a range of 10–40% during the initial 5 h of time on stream [25]. In contrast, SiO2-Al2O3 with the strong acidity showed the most serious catalytic deactivation. On the other hand, we found that Ag-modified TiO2 with hydrogen flow was effective for inhibiting the formation of coke and improving the catalytic stability.
Amorphous SiO2 is a porous material with weak acidity [[26], [27], [28]], and it is widely used as a catalyst support [[29], [30], [31]]. Recently, we found that SiO2 can be used as a catalyst for vapor-phase dehydration such as the dehydration of aldoximes to nitriles [26] and the cyclization of levulinic acid to angelica lactones [27]. In the dehydration, the catalysts with strong acidity such as SiO2-Al2O3 exhibited high initial activities, whereas strong acid sites took part in the side reactions to form oligomers, which poisoned the active sites and decreased the stability of catalytic activity. In contrast to the strong acid catalysts, the catalytic activity of SiO2 was usually low, while it was stable [26,27]. Over the weak acidic SiO2, also, high conversion levels can be achieved by increasing the contact time. It should be emphasized that the selectivity to the dehydration products over SiO2 can be maintained since no side reactions proceed even at high conversions. For example, in the cyclization of levulinic acid to angelica lactones performed at 250 °C [27], the selectivity to angelica lactones over SiO2 was ca. 90% at a conversion of ca. 50%, whereas that was only ca. 80% over SiO2-Al2O3 at the same conversion level due to the further oligomerization of angelica lactones.
In the present study, several solid catalysts including different kinds of SiO2 were investigated in the vapor-phase aldol condensation of butanal. This study aims to achieve a high selectivity to 2E2H with stable and high conversions using appropriate solid catalysts. Furthermore, the active sites on the SiO2 surface were investigated and discussed.
Section snippets
Samples
Four kinds of SiO2 (CARiACT Q-3, Q-6, Q-10, and Q-15 with mean pore diameters of 3, 6, 10 and 15 nm, respectively) samples were supplied by Fuji Silycia, Ltd., Japan. γ-Al2O3 (DC-2282) was supplied by Dia Catalyst & Chemicals Ltd., Japan. SiO2-Al2O3 (N631HN) was purchased from Nikki Chemical Co. Ltd., Japan. Monoclinic ZrO2 (RSC-HP) was supplied by Daiichi Kigenso Kagaku Kogyo Co. Ltd., Japan. 10 mol% Li2O-modified monoclinic ZrO2, denoted as 10-Li2O/ZrO2, was prepared by an impregnation
Aldol condensation of butanal over various solid catalysts
Table 1 shows the catalytic reaction results of the aldol condensation of butanal over solid catalysts such as SiO2-Al2O3, Al2O3, ZrO2, 10-LiO/ZrO2, and SiO2 (Q-10) where the reaction was performed at 200 °C and a catalyst loading of 0.5 g. 2E2H, butanoic acid, and 1-butanol were detected as the major products in all the reactions. Values of the conversion and the selectivity were averaged between 1–5 h. Scheme 1 shows the proposed formation route of each detected product. Aldol condensation of
Conclusions
In this study, aldol condensation of butanal to form 2E2H was performed over several solid catalysts. Although strong acidic catalysts such as SiO2-Al2O3 and Al2O3 were active, they were deactivated rapidly due to the significant accumulation of carbonaceous species on the catalyst surface. In contrast, SiO2 with a weak acidity showed a low activity, but the activity was the most stable because carbonaceous species was relatively difficult to be accumulated on the surface of SiO2. Both the pore
Acknowledgments
We thank Mr. Shota Tamaki, Ms. Mami Tsumura, Ms. Asuka Yabe, and Ms. Minami Iwakiri for experimenting with the catalytic reactions and TG analysis at Chiba University.
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