Introduction - Advances in Chemistry (ACS Publications)

Introduction 1

Downloaded by CARLETON UNIV LIBRARY on June 6, 2014 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch001

DONALD R. MARTIN Olin Mathieson Chemical Corp., Niagara Falls, N. Y.

As a consequence of events which occurred in the closing years of World War II, the need for an aircraft fuel containing more energy was recognized. Military operations were requiring greater efficiencies from their air-borne weapons in terms of either a greater distance or a larger pay load per unit of fuel. The high energy available in boron compounds was known and the British in 1947 evaluated some of these compounds for use as ramjet fuels. The United States Department of Defense, seeking a better fuel, initiated Project ZIP in 1952. The objective of ZIP was to develop a fuel having properties similar to JP-4 (a hydrocarbon jet fuel) but having a higher density, a fasterflamespeed, a lower vapor pressure, and more energy per pound. Project ZIP was large and had as its two prime contractors the Callery Chemical Co. and the Olin Mathieson Chemical Corp. Many universities, research institutes, government laboratories, and private companies were subcontractors on the project. After a careful evaluation of the high energy-containing, light weight elements, it was decided that the fuel objective of Project ZIP should contain a borane or a modified borane. Consequently, research in boron chemistry was pursued feverishly by the many scientists working on the project. To facilitate the exchange of experimental results obtained on these programs and to stimulate interest among those not on the project, the Division of Inorganic Chemistry of the American Chemical Society appointed a Committee on Boron Chemistry in 1957. This committee [composed of G. W. Schaeffer, St. Louis University (deceased), R. W. Parry, University of Michigan, and D. R. Martin, Olin Mathieson Chemical Corp., Chairman] sponsored a number of programs on boron chemistry at the national meetings of the American Chemical Society. Symposia were held at two of these meetings at which the papers in this book were presented: "From Borax to Boranes" in April 1958 in San Francisco, and "Boron and Binary Boron Compounds" in April 1959 in Boston. The relationship of these papers to the probable steps in making a High Energy Fuel (HEF) is self-evident in the following description of a composite of possible processes for going "From Borax (an ore) to Boranes (a fuel)." The three most common and stable boranes known at the inception of the project were diborane, pentaborane(9), and decaborane. Under ambient conditions, diborane is a gas, decaborane is a solid, and pentaborane(9) is a liquid possessing certain objectionable properties. It is pyrophoric and toxic, has a low density and a comparatively high vapor pressure. However, the heat of combustion (Table I) of each of these boranes was attractive and a logical procedure would be to attempt to modify these molecules sufficiently to give the desired properties without sacrificing the greatly desired high heat of combustion. Present address, 2714 Drummond Road, Toledo 6, Ohio. 1 1

In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

1

ADVANCES

Table I.

SERIES

Heat of Combustion of Fuels Fuel

B.t.u./Lb.

Hydrogen Diborane Pentaborane(9) Decaborane Methyldiborane JP-4


IN CHEMISTRY

51,571 31,078 29,070 27,850 26,420 18,400

After inspection of Table I , it is obvious that most of the heating value of a fuel comes from the captive hydrogen. Therefore, a high energy fuel should contain boron, as much hydrogen as possible, and a third element to modify its properties. The modifying element selected was carbon, inasmuch as it? was a component of hydrocarbon fuels and inasmuch as the materials, reactions, and techniques of organic chemistry could be utilized. T h e process of modifying the boranes would not be easy, as can be seen from Table I . F o r example, the replacement of one hydrogen atom i n a diborane molecule b y the smallest organic radical, the methyl group, results in a diminution of the heat of combustion b y at least 1 5 % (compare diborane with methyldiborane). Because of the sensitivity of the heat of combustion of a borane molecule to the replacement of hydrogen b y hydrocarbon groups, the ternary diagram (Figure 1) for boron, hydrogen, and carbon was constructed to serve as a guide for research. 5/jS7l

Figure 1.

©TU./LB.

Ternary diagram for boron, hydrogen, and carbon

I n the ternary diagram, the heats of combustion for known hydrocarbons and boranes were indicated on the appropriate axes. Recalling that the fuel should contain as much hydrogen as possible, and leaving one of the four sp "bonds of boron and of carbon for bonding, a tie line was drawn on the diagram from a point representing the composition of B H on the borane axis to a point representing the composition of C H on the hydrocarbon axis. Obviously, this line represents the maximum amount of hydrogen that the resulting fuel molecule can have. B y similar reasoning, and insisting that at least one hydrogen atom must remain attached to each boron and carbon atom i n the fuel molecule, a tie line was drawn from the composition representing B H on the boron-hydrögen axis to the point representing the composition 3

8

8


3

INTRODUCTION


of C H on the carbon-hydrogen axis. This tie line represents the minimum amount of hydrogen i n the synthetic fuel molecule. The next step was to plot on the diagram the points for the heats of combustion for known organoboranes. Isotherms (dashed lines) were then drawn through compositions whose heats of combustion were 25 and 5 0 % greater than J P - 4 . B y this procedure, a parallelogram was described o n the diagram, within which were compositions for possible organoboranes having heats of combustion which would be 25 to 5 0 % greater than J P - 4 . T h e objective of Z I P was then to synthesize molecules whose compositions fell within the area described b y the parallelogram. Figure 2 is a flow chart for a composite of processes for converting the ores of boron into a high energy fuel.

Na,B 0 • l O H j O - ^ H j B O j - ^ C H j O J a B - U N a B H - ^ B j H e 4

7

4

Figure 2.

High Energy Fuel

Borax to borane flow chart

The processes may be classified into four main steps: conversion of boron ores to boron halides or sodium borohydride, preparation of diborane from boron halides or sodium borohydride, conversion of diborane to higher boranes, and alkylation of boranes to organoboranes.

Conversion of Boron Ores to Boron Halides or Sodium Borohydride N o t only borax, but the other ores of boron, may be converted to boric acid by various processes involving the use of acids—e.g., sulfuric acid (step a) (1). Boric acid may be calcined to give boric oxide ( b ) . Boric oxide may be converted to borides or boron carbide b y heating with metals or carbon (c). These borides may be chlorinated to give boron trichloride (d) or treated with an acid such as hydrochloric acid, to give boranes—e.g., tetraborane (e). Boron trichloride also may be prepared from boric oxide b y chlorination i n the presence of carbon ( f ) . Trimethoxyborane is readily prepared b y esterification of boric acid with methanol (g). Sodium borohydride is prepared b y a reaction involving sodium and hydrogen or sodium hydride with boric oxide (h), trimethoxyborane (i), or boron trifluoride ( j ) . Boron trifluoride is readily prepared from boron ores or boric acid b y reaction with an acid—e.g., sulfuric acid—in the presence of a fluoride, or with hydrogen fluoride (k,l).


ADVANCES I N CHEMISTRY SERIES

4

Preparation of Diborane from Boron Halides or Sodium Borohydride Diborane, the key intermediate, may be prepared by the reaction of boron halides with active metal hydrides, aluminohydrides, or borohydrides (m,n,o). Boron t r i chloride may be hydrogenated i n the presence of an active metal (zinc, magnesium) (m) to produce diborane. Boron trifluoride may be converted by the Ziegler process to diborane (o). Sodium borohydride will yield diborane i f treated with a Lewis acid or a protonic nonoxidizing acid (n).

Conversion of Diborane to Higher Boranes


The higher boranes may be obtained by controlled pyrolysis of diborane

(p,q,r).

Alkylation of Boranes to Organoboranes The appropriate borane or boranes may be alkylated to produce the desired high energy fuel (s,t,u,v) b y a Grignard reaction or b y the reaction of an alkyl halide with a metalloborane or a metal alkyl with haloborane. The chapters which follow discuss the chemistry related to some of the problems involved i n making and handling a high energy fuel.

Literature Cited (1) Martin, D. R., J. Chem. Educ. 36, 208-14 (1959).


Introduction - Advances in Chemistry (ACS Publications)

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