Dehydration, catalyst

Reaction with Ammonia. Although the Hquid-phase reaction of acrolein with ammonia produces polymers of Htde interest, the vapor-phase reaction, in the presence of a dehydration catalyst, produces high yields of [ -picoline [108-99-6] and pyridine [110-86-4] n.2L mXio of approximately 2/1. 

Dimethyl Ether. Synthesis gas conversion to methanol is limited by equiUbrium. One way to increase conversion of synthesis gas is to remove product methanol from the equiUbrium as it is formed. Air Products and others have developed a process that accomplishes this objective by dehydration of methanol to dimethyl ether [115-10-6]. Testing by Air Products at the pilot faciUty in LaPorte has demonstrated a 40% improvement in conversion. The reaction is similar to the Hquid-phase methanol process except that a soHd acid dehydration catalyst is added to the copper-based methanol catalyst slurried in an inert hydrocarbon Hquid (26). 

Other processes recently reported in the Hterature are the gas-phase reaction of lactonitnle [78-97-7] with ammonia and oxygen in the presence of molybdenum catalyst (86), or the vapor-phase reaction of dimethyl malonate with ammonia in the presence of dehydration catalyst (87). 

C using a dehydration catalyst consisting of alurninosihcate, Al O, or siUca gel (45). 1-Naphthaleneamine is also toxic (LD q (dogs) = 400 mg/kg) and a suspected human carcinogen, which conditions mandate that appropriate precautions be followed in manufacture and use. 

In the third method adipic acid is converted to ADN via dehydroamination with NH in the gas (131) or Hquid phase (132) a dehydration catalyst, usually phosphoric acid, is used. 

Nitriles. Nitriles can be prepared by a number of methods, including ( /) the reaction of alkyl haHdes with alkaH metal cyanides, (2) addition of hydrogen cyanide to a carbon—carbon, carbon—oxygen, or carbon—nitrogen multiple bond, (2) reaction of hydrogen cyanide with a carboxyHc acid over a dehydration catalyst, and (4) ammoxidation of hydrocarbons containing an activated methyl group. For reviews on the preparation of nitriles see references 14 and 15. 

Amination. Isopropyl alcohol can be aminated by either ammonolysis ia the presence of dehydration catalysts or reductive ammonolysis usiag hydrogeaatioa catalysts. Either method produces two amines isopropylamine [75-31-0] and diisopropylamine [108-18-9]. Virtually no trisubstituted amine, ie, triisopropyl amine [122-20-3], is produced. The ratio of mono- to diisopropylamine produced depends on the molar ratio of isopropyl alcohol and ammonia [7664-41-7] employed. Molar ratios of ammonia and hydrogen to alcohol range from 2 1—5 1 (35,36). 

The butanols undergo the typical reactions of the simple lower chain aUphatic alcohols. For example, passing the alcohols over various dehydration catalysts at elevated temperatures yields the corresponding butenes. The ease of dehydration increases from primary to tertiary alcohol /-butyl alcohol undergoes dehydration with dilute sulfuric acid at low temperatures in the Hquid phase whereas the other butanols require substantially more stringent conditions. 

H-C-H H-C-H H-C-H Dehydration Catalyst plus heat Dehydrated castor oil... 

Dehydration catalyst com- Catalyst development partly substi- 63... 

In the above three processes, the catalysts are all composed of Cu-based methanol synthesis catalyst and methanol dehydration catalyst of AI2O3. The reactors used by JFE and APCI are slurry bubble column, while a circulating slurry bed reactor was used in the pilot plant in Chongqing. It can be foxmd from Table 1 that conversion of CO obtained in the circulating slurry bed reactor developed by Tsinghua University is obvious higher and the operation conditions are milder than the others. 

It will be shown that, upon interaction with water or ammonia, the T -like symmetry of the Ti(IV) centers in TS-1 is strongly distorted, as testified by UV-Vis, XANES, resonant Raman spectroscopies [45,48,52,58,64,83,84], and by ab initio calculations [52,64,74-76,88]. As in Sect. 3 for the dehydrated catalyst, the discussion follows the different techniques used to investigate the interaction. 

Fig. 1 compares the activities of vanadium-, cobalt- and nickel- based catalysts in ODH of ethane. Representative catalysts contained between 2.9 and 3.9 wt.% of metal. It is to be pointed out that metal oxide-like species was not present at any of the catalysts, as its presentation is generally the reason in the activity-selectivity decrease. The absence of metal oxide-like species was evidenced by the absence of its characteristic bands in the UV-Vis spectra of hydrated and dehydrated catalysts (not shown in the Figure). The activity of catalysts was compared (i) at 600 °C, (ii) using reaction mixture of 9.0 vol. % ethane and 2.5 vol. % oxygen in helium, and (iii) contact time W/F 0.12 g. i.s.ml 1. These reaction conditions represent the most effective reaction conditions for V-HMS catalysts [4] The ethane conversions, the ethene yields and the selectivity to ethene varied between 13-30 %, 5-16 %, and 37-78 %, respectively, depending on the type of metal species (Co, Ni, V) and support material (A1203, HMS, MFI). 

Dehydration is undesirable because a, -unsaturated carbonyls are catalyst inhibitors. To make matters worse, phosphines can add to the a, -unsaturated carbonyl (Equation 2.3) to give a product that is a dehydration catalyst, so the deactivation spiral continues. 

Whitmore (16), when developing the idea of carbonium ions, included reactions over dehydrating catalysts. The application of carbonium ion mechanism to the dehydration of alcohols over alumina has found several supporters (17, 18). 

Aluminas, which were prepared from sodium aluminate and which retained about 0.1 % of sodium ions, had a large amount of weakly acid sites, and were therefore excellent dehydration catalysts. At the same time these aluminas did not isomerize cyclohexene, owing to the absence of strong acid sites, which were neutralized by the alkali metal ions. Pines and Haag (36) determined that the upper limit of the total number of acid sites, capable of dehydrating butanol, and of the number of strong acid sites, capable of isomerization of cyclohexene, was 10 X 10 and 8 X 10 sites per cm, respectively. 

Metal molybdates421 and cobalt-thoria-kieselguhr422 also catalyze the formation of hydrocarbons. It is believed, however, that methanol is simply a source of synthesis gas via dissociation and the actual reaction leading to hydrocarbon formation is a Fischer-Tropsch reaction. Alumina is a selective dehydration catalyst, yielding dimethyl ether at 300-350°C, but small quantities of methane and C2 hydrocarbons423 424 are formed above 350°C. Heteropoly acids and salts exhibit high activity in the conversion of methanol and dimethyl ether.425-428 Acidity was found to determine activity,427 130 while hydrocarbon product distribution was affected by several experimental variables.428-432... 

Hundreds of substances of many types have been tested as dehydration catalysts and found active. Lists can be found in the literature [69,76,85] and we need to name here only such catalysts which show high activity and selectivity. The latter parameter is more important because a number of solids, especially oxides, can catalyse both the dehydration and the dehydrogenation of alcohols. The formation of aldehydes or ketones is then a parallel reaction to the dehydration, and the ratio of the rates depends on the nature of the catalyst. Only few oxides are clean dehydration or dehydrogeneration catalysts, but the selectivity may be shifted to some extent in either direction by the method of catalyst preparation. 

The important groups of dehydration catalysts are oxides, aluminosilicates (both amorphous and zeolitic), metal salts and cation exchange resins. Most work on mechanisms has been done with alumina. 

All recent results suggest the second mechanism. The arguments for its validity may be summarised as (1) high stereospecificity of elimination on a number of catalysts (2) existence of both basic and acidic sites on dehydration catalysts (3) the possibility of treating all elimination reactions in a common way from the point of view of mechanism (cf. Sect. 2.1). 

The best catalysts for olefin hydration are not necessarily those which have proved most satisfactory for the reverse reaction. Some of the successful hydration catalysts are not typical dehydration catalysts. The more obvious reasons are (i) different adsorption characteristics of the catalyst is desirable, e.g. stronger adsorption of olefin relative to alcohol, (ii) under the conditions used for the hydration, ether formation cannot be suppressed as readily as in the dehydration, (iii) at high pressures, the olefins tend to polymerise much more than at the low pressures used for the dehydration. 

QUELET REACTION. Passage of dry hydrogen chloride through a solution in inligroin of a phenolic ether and an aliphatic aldehyde in the presence or absence of a dehydration catalyst to yield (y-chloroalkyl derivatives by substitution in the para position to the ether group or in the ortho position in pom-substituted phenolic ethers. 

The catalytic reactions were performed either on the hydrated solids (equilibrated with the relative humidity -about 55%- of the atmosphere) or on the dehydrated catalysts (heated at 160°C, during 3 hours). An intimate mixture of the inorganic solid (100 mg) and the oxime in solid state (20 mg) was introduced in a Pyrex glass reactor. Thus, the reaction was carried out in "dry media" conditions, i.e. without any solvent. The mixtures were either activated with a microwave oven or heated at 100, 130 or 160°C in a conventional oven, during variable times (in the standard procedure 1 hour). The microwave oven used is a domestic (2450 MHz) Moulinex model FM 460, carrying out the experiments at 600 W of power and introducing a unic vessel in the oven in each experiment. The reaction products were extracted by treatment with a large excess (5 ml) of an appropriate solvent (methanol or chloroform), and the extracts were analyzed by GC. 

Dioxanc is produced by healing ethylene glycol in the presence of dehydration catalysts It finds wide u c j u solvent... 

Of commercial interest is the direct reaction of alcohols with ammonia at elevated pressures and temperatures in the presence of a dehydrating catalyst such as alumina gel. This process is known as ammonolysis and gives a mixture of primary, secondary and tertiary amines (Reaction XXVII). 

The most studied dehydration catalysts are the metal oxides (53), but the selectivity of these catalysts in terms of dehydration versus dehydrogenation is not fully understood (54). 

Still another multi-reactor approach is to divide the MTG reaction into two steps as shown in Figure 7. In the first step, methanol is partially dehydrated to form an equilibrium mixture of methanol, dimethyl ether and water over a dehydration catalyst. About 15% of the reaction heat is released in this first step. In the second step, this equilibrium mixture is converted to hydrocarbons and water over ZSM-5 catalyst with the concomitant release of about 85% of the reaction heat. Though this two step approach does not have any of the inherent complications of the previously mentioned multibed reaction systems, it leaves one with a substantial amount of the reaction heat (85%) still to be taken over one catalyst bed. This requires a fairly high recycle stream to moderate the temperature rise over the second reactor. Such a high recycle design would require careful engineering in order to transfer heat efficiently from the reactor effluent to the recycle gas and reactor feed. However, this two stage reactor system is the simplest of the fixed-bed systems to develop. 

Catalytic Experiment. The alkylation of meta-diisopropylbenzene with propylene was performed at 463 K in a flow-type fixed-bed reactor. The carrier gas nitrogen was first saturated with the vapor of meta-diisopropylbenzene (97 %, Aldrich) and then admixed with propylene (99 %, Matheson). The partial pressure of propylene and meta-diisopropylbenzene was 42.6 and 6.0 Torr, respectively. (The molar ratio of propylene and meta-diisopropylbenzene at the reactor inlet was 7.1 1). The modified residence time of propylene and meta-diisopropylbenzene W/Fpr0pyiene and W/Fm DiPB ranged from 4 to 20 and from 25 to 150 gh/mol, respectively, where W indicates the weight of dehydrated catalyst at 623 K and Fi indicates the molar flow rate of reactant i at the reactor inlet. The reaction conditions, viz. the reaction temperature, amount of catalyst, partial pressure and modified residence time of reactants, were chosen in order to obtain conversions of meta-diisopropylbenzene around 25 %. 

One of the most important variables in the TPD of CO from a supported Pt catalyst is the sample pretreatment. Calcination at 500°C for one hour followed by reduction is the conventional method to obtain the maximum exposed Pt and this follows closely to refinery practice for start-up and regeneration of commercial catalysts. The final step in our case was a 600°C He sweep for 30 minutes to ensure a fully dehydrated catalyst up to this temperature so that no water evolved during the subsequent TPD. We had previously observed that a high temperature He sweep could reduce the Pt catalyst without a prior H reduction presumably by the decomposition of the Pt oxide. 

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